Preheat oven to 400 degrees F. Spray both sides of the flour tortillas with cooking spray. Sprinkle a little bit of salt on each side of the tortilla. Using a fork, poke little holes all over the tortillas. This will prevent them from puffing up in the oven. Place on a large baking sheet and bake for 4-5 minutes, then turn tortillas over, and bake for 2 more minutes. Remove from oven and sprinkle cheese evenly on all of the tortillas. Place back and oven and bake just until cheese is melted, about 2 more minutes. Remove from oven and set aside.
To make the guacamole, cut the avocados in half and remove the seeds. With a spoon, scoop out avocado from the peel and put in a medium mixing bowl. Mash the avocado with a fork, leaving some chunks. Stir in the lime juice, red onion, cilantro, and tomato. Season with salt and black pepper, to taste.
Spread guacamole evenly on tostadas. Top with additional shredded cheese, if desired. Serve immediately.
Note-I didn't put jalapeño in the guacamole so the boys would eat it. If you want a kick to your guacamole, feel free to add jalapeño! For gluten-free use corn tortillas.