1. Make the salsa molcajete: Preheat oven to 350 degrees. Heat grill to 375 degrees (medium-high heat) and char tomatillos, tomatoes, onions, and jalapeños, then place in an oven-safe pan and cover ¾ the way with water. Cover with foil and roast in oven for 30 minutes.
2. Carefully transfer contents from roasting pan to a blender and blend until chunky. Add garlic and salt and continue to blend. Transfer to a bowl and set aside.
3. Make the carne asada: In a blender, combine first seven ingredients and puree until smooth. Pour over sirloin and marinate in refrigerator for 3 to 4 hours. Heat grill to 375 degrees (medium-high heat) and grill meat for 6 to 8 minutes per side for medium.
4. While steak cooks, make the guacamole: In a bowl, mash avocado, then stir in remaining ingredients. Add salt to taste, cover, and set aside.
5. To assemble: After steak has rested for 5 minutes, slice very thin and set aside. Place tostadas on a sheet tray and layer with steak and cheese, then broil until cheese has melted. Move to a serving platter and garnish with salsa, guacamole, and cilantro.