Place onion, chili pepper, and cilantro in a bowl.
Scoop out the pulp of avocado and add it to the bowl.
Smash avocado with a fork and mix with all ingredients.
Season with salt and lime juice to your liking and set aside.
Make carne asada
Season beef with salt, pepper, and a pinch of cumin.
Heat a stove grill pan or skillet on medium-high heat until smoking hot.
Add a drizzle of olive oil to lightly coat the pan and sauté the meat until is no longer pink and is cooked through (about 8 minutes).
Turn off the heat and cover the pan to prevent the meat from drying.
Assemble mulitas
Heat a skillet or pan over medium heat and drizzle with a little bit of oil.
Place a tortilla and top with cheese and meat.
Add more cheese on top and close with another tortilla.
Allow the cheese to start melting, then carefully, using a spatula, flip over to cook the other side. Gently press the tortilla to stick it with cheese and meat.
Keep flipping the mulitas until cheese is melted and bubbling and the tortillas are a little bit crispy.
Put mulitas on a plate, gently peel the top tortilla and add guacamole and pico de gallo. Close and serve immediately.
Notes
Cut the meat into thin slices and then small bites. This allows for quick cooking.
Don't overcook the meat as it can get tough/hard, once done, remove from heat and keep it covered to prevent it from drying out.