From Sicily

Fresh tuna, one slice per person about 1" thick
1/2 cup balsamic vinegar
Salt & pepper to taste
1 cup fresh mint leaves
2 tsp. dried oregano
2 large onions, sliced thin
Extra-virgin olive oil

Combine the vinegar, salt, pepper, mint leaves and oregano in a non-reactive pan. Marinate the tuna slices in the liquid for 15 minutes. Prepare a large piece of aluminium foil for each slice of fish, by brushing the foil with oil and lining it with sliced onions. Drain the fish and place each slice on a piece of aluminium foil. Cover with another layer of onions. Tightly seal the foil around each slice of fish and cook on the grill for 10-15 minutes per side, or in a 350-degree oven for 15 minutes.