Pat the galletto pieces dry with paper towels and season them with 1 1/2 tsp salt and 1/2 tsp black pepper. Dry skin browns, wet skin steams.
Heat the olive oil in a 30 cm / 12-inch deep saute pan over medium-high heat until it shimmers. Brown half the pieces skin side down for 5 minutes, turn them, and brown 3 minutes more until deep golden. Move them to a plate and repeat with the remaining pieces. They don't need to cook through.
Pour off all but 2 tbsp of fat and lower the heat to medium. Add the onion, carrot and celery with a pinch of salt and cook for 6 minutes, stirring, until soft and pale gold. Add the bell pepper and mushrooms and cook for 3 minutes. Stir in the garlic and tomato paste and cook for 1 minute.
Pour in the wine, scrape up the browned bits from the bottom, and simmer for 2 minutes until reduced by about half. Add the crushed tomatoes, stock, rosemary, sage and bay leaf. Return the chicken and any plate juices to the pan, skin side up, nestling the pieces into the sauce, and bring to a bare simmer.
Cover the pan and lower the heat so only a few bubbles break the surface. Simmer for 30 minutes, until the thickest thigh reads 165 F / 74 C and the meat is tender. Don't let it boil, or the breast meat will tighten.
Uncover, stir in the olives, and simmer for 10 minutes until the sauce coats the back of a spoon. Remove the rosemary stems, sage and bay leaf, then take the pan off the heat. Cover and rest for 10 minutes, taste for salt, and serve.