Heat the oven and set up. Heat the oven to 375°F (190°C) with a rack in the middle. Line an 18x13-inch half sheet pan with parchment paper – parchment, never wax paper. Tear off a second sheet of parchment and lay it on the counter as your assembly surface. Read the next two steps before you start: once the shells come out of the oven you have a two- to three-minute working window, and there is no time to go looking for anything.
Make the filling. In a medium bowl, beat the softened cream cheese with 2 tablespoons of the marinara, 1/2 teaspoon of the oregano, the garlic powder and the black pepper until completely smooth and no orange streaks remain – you want the sauce emulsified into the cheese, not sitting on top of it. Fold in 1/4 cup of the shredded mozzarella and the chopped basil. Taste, then add a pinch of salt only if it needs it; the cheeses and turkey pepperoni are already salty. You will have about 1 cup of filling.
Portion everything before the shells go in. Divide the filling into 4 equal mounds – a scant 1/4 cup, about 3 3/4 tablespoons each – on a plate. Separate the 8 turkey pepperoni slices into 4 pairs. Chop the parsley. Set the plate next to your sheet of counter parchment. This is the step that decides whether the last two shells crack, so do not skip ahead.
Shape the shells. Divide the remaining 1 3/4 cups shredded mozzarella into 4 equal mounds of 7 tablespoons (a scant 1/2 cup) each on the lined pan, arranged two by two with at least 2 inches between them. Spread each mound into an even rectangle about 5 x 3 inches – deliberately smaller than the finished shell, because they spread as they melt and will come out at roughly 6 x 4 inches. Keep each one an unbroken single layer with no bare patches; any gap becomes a tear later. Sprinkle the remaining 1/2 teaspoon oregano evenly over the four rectangles.
Bake the shells at 375°F for 12 to 14 minutes – start checking at 10, and let the cue decide rather than the clock. At 7 tablespoons of cheese each these sheets are far heavier than the thin rounds most cheese-shell recipes bake, and a heavier sheet needs longer to shed its water; 8 minutes leaves them wet in the middle, and a wet sheet never sets. They are ready when the bubbling in the center has stopped: the edges may still simmer faintly, but the middle should look still, matte and lacy with tiny pinholes rather than wet and glossy, and the edge should be a continuous deep gold the whole way round. Do not wait for the center to brown – a sheet browned in the middle has gone past pliable and will snap instead of bending. While they bake, warm the remaining 6 tablespoons of marinara for dipping (30-40 seconds in the microwave, or a minute in a small pan).
Cool to the pliable window – on the pan. This is the step that makes the whole recipe work, and the hot pan is part of it: leave the parchment on it and do not slide the sheets onto the counter or a rack. The pan goes on driving moisture out of the underside for another minute or two and holds the cheese warm so the window stays open, where a cold surface chills the sheets from both sides and can take one from floppy to brittle in under a minute. Blot the pooled oil off the top with a paper towel – a 7-tablespoon sheet renders a lot of it, and oil left sitting there stops the seam gripping – then rest 2 minutes, 3 at the outside. Test one before you commit: push the edge with a thin spatula. It should slide across the parchment as a single piece, and the lifted edge should hold a curve without flopping or snapping – it feels like warm leather. If it stretches like the cheese on a hot pizza slice, or leaves a greasy film glued to the paper, it is underbaked: every sheet is underbaked together at this stage, so put the whole pan back in for 1 to 2 more minutes. If it clicks against the paper or cracks as you lift it, it has cooled too far. Work through them one at a time, in order, and expect the third or fourth to stiffen while you are filling the first two – that is normal, not a mistake. When one does, re-soften that single shell and nothing else: slide a thin spatula under it, lift it onto a square of parchment or foil set on a second pan, and give it 30 seconds in the oven on its own. Do not put the whole sheet pan back in – the shells you have not got to yet are still inside their window, and a second blast of heat would re-melt the ones that were perfectly fine.
Fill. Spread one portion of filling over each shell in an even layer a scant 1/4 inch (about 5 mm) thick, taking it right out to within 1/4 inch of both long edges but leaving 1/2 inch bare at each short end. The filling itself is what seals this roll – cream cheese is the glue, exactly as it is in a tortilla pinwheel – so do not leave a wide bare strip along the edge you are rolling toward. The bare 1/2 inch at each end is there for the opposite reason: it gives the filling somewhere to go when you close the roll instead of squeezing out. Lay 2 slices of turkey pepperoni end to end over the filling along the near long edge.
Roll. Starting from the turkey pepperoni edge, roll the shell away from you into a snug cylinder – snug enough that there is no hollow down the middle, but do not squeeze or compress it, or you will push filling out of both ends. Use the parchment underneath to lift and guide the roll while keeping the paper itself out of it. Finish with the seam underneath. Repeat with the remaining three shells to make 4 roll-ups total.
Rest, then serve. Leave the roll-ups seam-side down for 3 minutes after the last one is rolled (the first ones will have had longer). They are ready when the outside feels firm and dry rather than greasy and the seam stays shut when you lift one. Do not return the filled roll-ups to the oven – the shells are already baked, and a second bake would only soften the cream cheese and let it run out the ends. Scatter the parsley over the top and serve them warm and whole, with the warmed marinara alongside for dipping. They are warm rather than hot: the shell still carries oven heat and the filling is at room temperature, which is as hot as this dish can safely get. Do not try to slice them at this stage – for a pinwheel platter, see the note on chilling.