1. Preheat oven to 350°.
2. Heat a large nonstick skillet over medium-high heat. Sprinkle chicken with 1/4 teaspoon salt and black pepper. Add 2 teaspoons oil to pan; swirl to coat. Add 4 chicken thighs to pan; cook for 4 minutes on each side or until browned. Remove chicken from pan, and keep warm. Repeat procedure with 2 teaspoons olive oil and remaining chicken.
3. Add the remaining 2 teaspoons olive oil to pan, and swirl to coat. Add bell peppers, 1 cup onion, celery, rosemary, minced garlic, and remaining 1/8 teaspoon salt; cook for 3 minutes, stirring frequently. Add mushrooms; cook for 6 minutes, stirring occasionally. Stir in marinara sauce; cook 1 minute.
4. Spread vegetable mixture in bottom of a 13 x 9-inch glass or ceramic baking dish. Arrange chicken on top of the vegetable mixture. Bake at 350° for 30 minutes or until a thermometer inserted into chicken registers 160°. Sprinkle with cheese.