1Preheat the oven to 350 degrees F (175 degrees C).
2Heat olive oil in a large Dutch oven over medium-high heat; add chicken and cook until browned on the outside. Remove to a bowl to capture the juices.
3Stir in onions and mushrooms; cook for 5-6 minutes until soft. Add a big pinch of salt and pepper. Stir in garlic, rosemary, red pepper flakes, oregano, tomato sauce, and water.
4Place chicken pieces and any juices that have accumulated in the bowl on top of the cooked vegetables. Add more salt and pepper. Place pepper slices on top of the chicken.
5Cover and cook in the preheated oven for 1 hour 15 minutes.