Season chicken thighs with salt and pepper.
In a large, deep skillet with lid (I'm using this 3 ½ qt nonstick, but a Dutch Oven would work too) heat a few Tbsp olive oil over med high, sear chicken thighs skin-side down for 5 mins until browned. Remove from pan, set aside.
In the same skillet, add olive oil then sliced onion, peppers , garlic, & chili flakes. Sauté for 5-8 minutes over medium heat until softened and beginning to take on color.
Add halved Castelvetrano olives and pureed whole peeled tomatoes to the skillet. Stir and bring to a simmer.
When simmering, return the seared chicken thighs to the skillet, nestling them into the sauce. Cover and braise in a preheated 325°F (165°C) oven for 45 minutes.
Serve chicken over stewed peppers, onions, and olives.