Seared Mackerel with Marinated Tomatoes

Name: Seared Mackerel with Marinated Tomatoes
Ingredients: 2 pounds skin-on mackerel fillets, pin bones removed, cut into 8 equal portions, skin side scored; 3/4 teaspoon fine sea salt, divided; 1/2 teaspoon black pepper, divided; 3 tablespoons extra-virgin olive oil; 6 tablespoons fresh lemon juice (from 3 lemons), divided; 7 tablespoons olive oil, divided; 1 pound heirloom tomatoes (about 2 medium tomatoes); 1 tablespoon white balsamic vinegar; 2 tablespoons fresh lime juice; 2 medium shallots, finely chopped; 1 small bunch fresh chervil or flat-leaf parsley, stemmed and finely chopped