In a medium saucepan, heat 1 tablespoon of olive oil. Add the onions and cook over low heat until soft but not brown, 8 to 10 minutes. Add the white wine, tomatoes and mint. Cook over low heat until reduced to ½ cup of liquid, 15 to 20 minutes. Add the clam juice, salt and pepper. Bring to a boil, then simmer for 20 to 25 minutes or until reduced by half. (The sauce can be prepared ahead and stored, covered and refrigerated, for up to three days.)

Dust the tuna lightly with flour, shake off the excess, and sprinkle with salt and pepper. In a sauté pan large enough to hold the tuna in a single layer, heat the 2 tablespoons of olive oil. Add the tuna and cook over medium heat, turning occasionally, until just lightly colored on all sides, 3 to 4 minutes. Add the tomato-onion sauce and simmer until the tuna is done but still pink inside, 4 to 5 minutes. Serve at once.

Makes 8 servings.