Marinate steak with the juice from 1/2 lime for about 20 minutes (but not longer than 30 minutes).
Preaheat oven to 350 degrees.
To make the avocado sauce, just combine all ingredients in a blender and mix until creamy. You may need to add more vegetable oil depending on how big your avocado is.
This part is of course optional, but I like my flour tortillas to be a little charred. Turn your stove top on medium heat. Doing each tortilla one at a time, use tongs to place it directly over the flame. It shouldn't take longer than about 30 seconds on each side. I like to move them around a bit so they get charred all over (and not just the edges).
Pat your steak dry and then liberally season both sides with salt, pepper, onion power, and paprika. If you want it a little spicy you can also add a little cayenne pepper here as well. Using your hands, gently rub the dry ingredients into the steak.
Heat a grill or cast iron grill pan on high and sprinkle with some vegetable oil. Once hot, grill on each side for approximately 4-5 minutes. Remove from the heat and let rest for 10 minutes (this part is important of your steak will lose its juiciness and bleed all over your tortillas!).
While the steak is cooking, line approximately two baking sheets with the tortillas and place a slice of cheese on each tortilla. Bake in the oven until the cheese is melted (about the time that your steak is cooking).
After steak has rested for 10 minutes, sliced into thin strips OR into cubes--whichever you prefer. Top tortillas with steak, avocado sauce, diced white onions. You can also garnish with more cilantro, squeeze more lime juice on top, and/or hot sauuuuuce!