In a 9 by 13-inch glass baking dish, combine the butter, tequila, thyme, serrano, garlic and cilantro and mix well. Transfer steak to the marinade and sprinkle with salt and freshly ground black pepper. Let stand at room temperature for 30 minutes or in the refrigerator for up to 2 hours.
Preheat a large griddle or skillet over medium-high heat and brush with olive oil. Cook the steak, turning occasionally, 6 to 8 minutes per side for medium-rare. Let rest on a plate covered with aluminum foil for 10 minutes. Cut the meat against the grain into 1/2-inch pieces.
On the same griddle used to cook the steak, over medium-high heat, place 3 (1/4 cup) mounds of cheese spaced 2 inches apart. Let cook for 3 minutes (the cheese will spread flat like a tortilla, make sure they don't stick together). After the cheese browns on the bottom (the cheese will stay white on the top side), add 1/4 cup sliced meat over half of the cheese and fold the other half over like a half moon-shaped quesadilla. Transfer to a baking sheet and cover with aluminum foil to keep warm. Repeat the procedure with the remaining ingredients.
Serve each cheese taco with sliced avocado and salsa or hot sauce.