In a stock pot, combine the vinegar, onion, garlic, bay leaves, peppercorns, add 2 quarts of water and cook over high heat until it is reduced by one-third. Add the tuna, reduce to a simmer, and cook for 45 minutes. Drain the tuna and place in a jar or terra cotta pot. Cover with olive oil and seal. Let sit for 20 to 30 days, refrigerated, then serve this perfectly preserved tuna with fresh bread and chopped red onion.