Preheat oven to 160°C. Check offal has been removed from poussin and that the cavity is clean. Wrap mirepoix in muslin cloth secured with string. Transfer mirepoix into a large casserole dish and place poussin on 24 of mirepoix. Pour over the stock, cover with a lid and bake in oven until the poussin is cooked through, about 30 minutes
Peel asparagus and baby onions. Wash and scrub carrots and baby turnips to remove any dirt. Discard outer leaves of cabbage, cut into quarters, remove and discard core and separate leaves. Bring salted water to a boil in a saucepan. Separately boil each type of vegetable until just soft, transferring to iced water each time; about 5 minutes for the turnips, carrots and cabbage, 3 minutes for the asparagus, 2 minutes for the fava (broad) beans and 1 minute for the peas. Drain, refresh and set aside. Gently squeeze the broad beans from their skins and discard the skins.
Once the poussin is cooked, remove the casserole dish from the oven and discard mirepoix. Remove cooking liquid by straining through a fine sieve into a clean pan leaving the poussin in the casserole dish. Add the tarragon, thyme and bay leaves to the poussin, re-cover with a lid to keep warm and allow the meat to rest and the herb flavors to infuse, about 10 minutes. Meanwhile reduce the cooking liquid over a high heat to concentrate the flavor, about 5 minutes and season. Add all the vegetables into the hot concentrated stock and continue to cook until reheated.
Return the hot cooking liquid and vegetables to the casserole dish with the poussin and herbs. Serve in casserole dish.