Marinate the Steak: Season the steak with salt, pepper, and garlic powder. Mix olive oil, lime juice, cumin, smoked paprika, and cilantro in a bowl. Marinate the steak in this mixture for at least 1 hour.
Cook the Steak: Grill or pan-sear the steak to your desired doneness, let it rest for 5 minutes, then slice thinly.
Prepare Nachos: Preheat your oven to broil. Layer tortilla chips on a baking sheet, sprinkle with half the cheeses, add steak slices, then top with remaining cheese.
Broil: Place the nachos under the broiler until the cheese is bubbly and golden, about 3-5 minutes.
Add Toppings: Remove from oven and immediately top with tomatoes, green onions, cilantro, jalapeños, and avocado or guacamole.
Serve: Serve hot with sides of sour cream and salsa.