Rib Eye Aguachile

Name: Rib Eye Aguachile
Ingredients: 1/2 red onion, thinly sliced (about 1 cup); 3 tablespoons, plus 1 tsp. olive oil, divided; 2 tablespoons red wine vinegar; 2 3/4 teaspoons kosher salt, divided; 1 teaspoon freshly ground pepper, divided; 1 (1-pound) boneless rib-eye steak (1 1/3-inches thick); 1/2 cup fresh lime juice; 3 tablespoons fresh orange juice; 2 teaspoons soy sauce; 1 (1/8 ounce) serrano chile, seeds and ribs removed, finely chopped (about 1 1/4 teaspoons), plus thin slices for serving; 1 garlic clove, finely chopped; 1 medium-size ripe avocado, chopped (about 3/4 cup)