Brown meat on both sides for 2-3 minutes in a dry cast iron pan. Season with salt and pepper and drizzle with 2 tablespoons oil. Wrap beef in aluminum foil and place in a preheated 80°C (approximately 175°F) for a few minutes.
Peel avocado, remove pit and dice finely. Rinse tomatoes, cut in half, remove seeds and dice. Peel garlic and chop finely. Peel onion and chop very finely. Mix avocado, tomato, garlic, onion and parsley in a bowl and season with plenty of pepper, salt, 1-2 teaspoons Tabasco, parsley, lime juice and 1 tablespoon oil. Let rest at least 10 minutes.
Peel white onions, cut in half, and cut into wide strips. Heat 2 tablespoons oil in a pan, saute onion strpis until golden brown and season with salt and pepper.
Heat tortilla for a few minutes in a preheated 250°C (approximately 475°F) oven. Cut beef into strips. Cover tortillas with onion strips, beef and avocado salsa to taste. Serve immediately.