Preheat the oven to 375°. In a blender, puree the onion with the garlic, red bell pepper, tomatoes and chicken stock.
Season the chicken with salt and pepper and dust with flour. In a large, deep, ovenproof skillet, melt the butter in the olive oil. Add the chicken and cook over moderately high heat, turning a few times, until well-browned, about 12 minutes. Transfer the chicken to a plate.
Add the onion-tomato puree, the bay leaf and rosemary to the skillet and bring to a boil, scraping up any browned bits stuck to the bottom of the skillet. Add the chicken pieces to the sauce, skin side up. Transfer to the oven and roast, uncovered, for 45 minutes, until the chicken is tender and the sauce is reduced by half. Discard the bay leaf and rosemary sprig and serve the chicken with noodles or crusty bread.