Keto Green Bean Casserole
Keto green bean casserole with a real crispy onion topping, not a soft almond flour streusel. The onions are baked separately and cooled before they go on, and the mushrooms and beans are dried out before assembly so the sauce never goes watery. 5.9g net carbs.
Equipment
- Mandoline slicer strongly advised for even thin onion slices
- Rimmed baking sheet
- Large skillet
- 2-quart baking dish
Ingredients
For the Crispy Onion Topping
- 1/2 medium yellow onion 85 g, sliced paper thin
- 1 tsp baking powder
- 1/4 cup finely grated Parmesan cheese 25 g
- 1/2 tsp fine salt
- olive oil spray spray generously
For the Casserole
- 1 lb fresh green beans 450 g, trimmed and halved
- 2 tbsp unsalted butter 28 g
- 10 ounces white button or cremini mushrooms 280 g, quartered
- 1 tsp fine salt divided
- 1/4 tsp black pepper
- 2 cloves garlic minced
- 1/4 tsp ground nutmeg
- 1 1/2 tsp arrowroot powder
- 1 cup chicken broth 240 ml, low sodium
- 3/4 cup heavy cream 180 ml
Instructions
Crispy Onions (do these first)
- Coat and bake: Heat the oven to 375°F and line a sheet with parchment. Slice the onion as thinly as possible, ideally on a mandoline. Toss with the baking powder, Parmesan and salt until every strand is dusted, spread out with minimal overlap, and spray generously with olive oil. Bake 18-22 minutes, tossing every 6 minutes, checking closely from 12 minutes - Parmesan browns much faster than you expect.
- Cool completely: Slide the parchment onto a rack and leave the onions until they are at room temperature and rattle when you tip the sheet. Warm onions carry steam that condenses inside the crust and softens them before they ever touch the sauce. Do not skip this.
The Casserole
- Blanch and shock the beans: Bring a large pan of salted water to the boil and set a bowl of ice water beside it. Boil the beans 2-3 minutes until bright green and still snappy, drain, and tip straight into the ice water. Drain again thoroughly and pat dry.
- Cook the mushrooms dry: Melt the butter in a large skillet over medium heat. Add the mushrooms with 1/2 teaspoon of the salt and the pepper and cook about 5 minutes, until they release their liquid and that liquid cooks away again. Add the garlic and nutmeg and cook 1 minute more.
- Build the sauce: Sprinkle the arrowroot dry over the mushrooms and stir to coat - this is what stops it clumping. Pour in the broth while stirring, simmer 1 minute, add the cream and cook 6-8 minutes until it coats the back of a spoon. Stir in the blanched beans and the remaining 1/2 teaspoon salt.
- Assemble and bake: Heat the oven to 400°F. Tip everything into a 2-quart baking dish, scatter the cooled onions over the top, and bake 15 minutes until hot and bubbling at the edges. If the onions came out very dark, hold them back and scatter them on after baking instead.
Notes
Arrowroot vs xanthan gum: xanthan works, but the working band for a sauce is about 1/8 to 1/4 tsp per cup of liquid. The half-teaspoon doses common in keto recipes are roughly double the top of that band and produce a slippery texture. Arrowroot has a wider margin. The 1 1/2 tsp used adds about 4g carbohydrate to the whole dish, around 0.5g per serving, already counted in the nutrition below.
Frozen green beans: cook per the packet, drain very well, pat dry, skip the blanching, and add an extra 1/2 tsp arrowroot - freezing ruptures the cell walls so they release more water. Canned beans are not recommended; they are fully cooked already and go mushy.
Sauce still thin? Return the baked casserole to a skillet over medium heat for a few minutes, or whisk another 1/2 tsp arrowroot into 1 tbsp of cold broth and stir it in. Never add dry arrowroot straight to hot liquid.
Make ahead: sauce keeps 2 days, beans can be blanched a day ahead, onions bake a day ahead and keep airtight at room temperature - never the fridge. Assemble without the onions up to a day ahead.
Storage: 5 days refrigerated, 3 months frozen without the onions. Reheat 350°F covered 20-25 minutes; re-crisp leftover onions in an air fryer at 360°F for 2-3 minutes.