Pickle red onions, cover, and reserve until needed.
Season the steak, grill or char, and let rest.
Stir lime juice, orange juice, soy sauce, serrano, garlic, salt, pepper, and olive oil.
Reserve ¼ cup of marinade.
Slice the steak and place it on a baking dish. Pour ¼ cup of the marinade over the steak. Cover and chill 1 hr.
Remove onions from pickling liquid.
Place the marinated steak on a platter. Drizzle with ¼ cup of the marinade. Garnish with pickled onions, avocado, and sliced serrano. Sprinkle with salt, and serve with tostadas.