Put the boar in a large bowl and cover with 4 cups of the wine. Toss in 3 of the bay leaves, the sage, rosemary, juniper berries and peppercorns. Cover with plastic wrap and refrigerate overnight.
The next day, heat a wood-fired oven to a moderate temperature. Alternately, preheat the oven to 300 degrees F.
Transfer the meat from the red wine marinade to a large Dutch oven; discard the marinade. Add the remaining 2 bay leaves, the tomatoes, olive oil, red onion, carrot, celery, the remaining 2 cups wine and big pinches of salt and pepper. Roast until the boar is tender, about 4 hours.