Heirloom Tomato Salad with Tuna Confit

Name: Heirloom Tomato Salad with Tuna Confit
Ingredients: 4 1/2 cups extra-virgin olive oil; 3 thyme sprigs; 1 bay leaf; 1 small fennel bulb, thinly sliced; 1/2 small carrot, thinly sliced; 1 1/2 teaspoons coriander seeds; 1 1/2 teaspoons black peppercorns; 1 teaspoon white peppercorns; 2 small yellow onions, thinly sliced; 1 pound tuna steak; 2 beefsteak tomatoes, sliced 1/4 inch thick; 2 ripe green heirloom tomatoes, such as Green Zebra, sliced 1/4 inch thick; 1/4 cup sherry vinegar; 1 1/2 teaspoons fresh oregano, large leaves torn; 1 1/2 teaspoons lemon thyme; 6 large basil leaves, torn; 6 large mint leaves, torn; Kosher salt; Pepper