Cut the chicken thighs into uniform 1-inch pieces — even sizing is what stops some pieces overcooking while others are still raw. Pat them completely dry with paper towels; this is what lets them sear instead of steam.
In a medium bowl, combine the salt, cracked pepper, Italian seasoning, garlic powder, onion powder and paprika. Add the chicken and toss until every piece is coated. Let it sit about 5 minutes while you prepare everything else.
Heat 1 tablespoon of the oil in a large skillet or cast iron pan over medium-high heat. When it shimmers, add the chicken in a single layer — work in two batches rather than crowding the pan. Cook 2–3 minutes without moving it, flip, and cook another 2–3 minutes. It should be golden on all sides but need not be cooked through yet. Transfer to a plate. Do not clean the pan: the browned bits are the base of the sauce.
Add the remaining tablespoon of oil and 1 tablespoon of the butter to the same pan. Once the butter foams, add the sliced mushrooms and leave them undisturbed for about 2 minutes to colour. Then stir and cook another 2–3 minutes until caramelised and most of their moisture has cooked off.
Add the shallot and green bell pepper and cook about 1 minute, stirring, until fragrant and translucent. Add the minced garlic and cook just 30 seconds — it burns fast.
Add the remaining 2 tablespoons of butter and let it melt. Return the chicken and any juices from the plate, and toss to coat. Lower the heat to medium and cook 3–4 minutes more, stirring occasionally, until the chicken is cooked through (165°F internal). Remove from the heat, sprinkle with the chopped parsley and serve straight away.