To make the guacamole, add the avocados, serrano peppers, cilantro, garlic, salt, and a bit of water to a blender. Blend until smooth, adding water as needed to thin it down. Transfer to a bowl and set aside.
To make the red sauce, add the dried chilies, stewed tomatillos in 2 cups (480 ml) of their stewing liquid, stewed tomato in ½ cup (120 ml) of its stewing liquid, garlic, and bit of salt in a blender. Blend until everything is well incorporated, adding more salt to taste.
Mix 1 teaspoon of salt into 1 tablespoon of taco seasoning and rub the seasoning onto both sides of the steak.
Cut the steak into thin strips, then cut into small pieces.
Add 1 tablespoon of oil to a large griddle or cast iron pan over high heat. Once the oil is hot, add the steak and cook for 6-8 minutes or until the steak has browned on all sides. Move to the side of the griddle to keep warm.
Reduce the heat to medium/medium-high and add a bit more oil to the griddle. Place the tortillas on the griddle and top with a generous amount of cheese. On half of the tortillas, add some of the cooked steak, diced onion, cilantro, red sauce, and guacamole.
Place the other tortilla (cheese-side down) on top of the fillings. Serve.