Grilled Salmon with Tomato-Red Onion Relish Recipe

Ingredients 1/2 small red onion, vertically sliced 4 (6 oz) salmon fillets 1 tablespoon olive oil 1/2 teaspoon kosher salt, divided 3/8 teaspoon black pepper, divided 1 pint cherry tomatoes, quartered 1 tablespoon capers,