Enchiladas Rib Eye with Aguachile

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)

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