How to Make It
Toss together onion, 2 tablespoons of the oil, vinegar, 1/2 teaspoon of the salt, and 1/4 teaspoon of the pepper in a medium bowl. Cover and chill until ready to serve.
Heat a 12-inch cast-iron skillet over medium-high. Season steak all over with 1 1/2 teaspoons of the salt and 1/2 teaspoon of the pepper. Add 1 teaspoon of the olive oil to skillet; swirl to coat. Add steak and cook, undisturbed, until a brown crust forms, about 5 minutes per side. Transfer steak to a cutting board, and let rest 10 minutes.
While steak rests, stir together lime juice, orange juice, soy sauce, chopped serrano, garlic, remaining 3/4 teaspoon salt, 1/4 teaspoon pepper, and 1 tablespoon olive oil. Reserve 1/4 cup marinade.
Slice steak against the grain; transfer to a baking dish and pour remaining marinade over steak. Cover and chill for 1 hour.
Remove onions from pickling liquid. Remove steak from marinade, and transfer to a serving platter. Drizzle with 1/4 cup of the reserved marinade. Garnish with pickled onions, avocado, and sliced serrano. Sprinkle with flaky sea salt. Serve with tostadas.