While your oven heats, prepare the veggies for the tostadas. Mash the avocado and mix the avocado with the lime juice. Slice the lettuce into thin strips and chop the onion and tomato into small pieces.
Lay the cheese slices out on baking sheets that are lined with parchment paper. Keep at least an inch of space between each slice of cheese.
Bake the cheese slices at 350 degrees until they are brown around the edges and light golden brown in the middle. This should take about 10 minutes.
Let the tostadas cool for a few minutes after you take them out of the oven, then top the tostadas with the veggies. Sprinkle salt and pepper on top of the tostadas.