Preheat oven to 425°F. Arrange chicken on a baking sheet, toss with crushed garlic, rosemary, about 2 tablespoons olive oil, salt and pepper. Roast about 45 minutes until cooked through and crispy at edges. Cool meat to handle then pull into small pieces, discarding the skin and bones. Reserve the bits of rosemary or garlic that remain with chicken; don’t fuss over separating them.
Warm the stock with the dried porcini in small pot.
Heat deep skillet or a Dutch oven with olive oil, 2 turns of the pan, over medium to medium-high heat. Add celery, carrot, onions, salt, pepper and bay leaf; stir and partially cover 5 minutes. Add chopped garlic, stir a minute, then add wine and reduce by half.
Drain mushrooms (if using), chop and add to sauce.
Add stock to sauce, reserving the last few spoonfuls where grit may have settled.
Add tomatoes to sauce, breaking up with hands or spoon. Add reserved chicken back to ragu and simmer sauce 30 minutes at low bubble. Remove bay leaf.
Serve with gnocchi or any sturdy, short-cut pasta such as rigatoni.