Place the tuna in an ovenproof casserole or Dutch oven and add the court bouillon and enough water to cover the fish completely. Bring to a boil. Reduce heat and simmer 10 minutes. Remove from heat , cover and allow to go completely cold in the cooking liquid. Drain off the cooking liquid and dry off the tuna. Place it in a large dish and add the peppercorns, salt, olive oil, lemon, onions, and thyme. Cover with plastic and marinate for several hours, turning the tuna 2-3 times. Serve with the black olives, hard-boiled eggs, and potato salad or hot boiled new potatoes.
