Note: you can bake or microwave this. Make sure that, if you’re using the microwave, you make it in a microwave safe pan.
Stir together chili and tomato sauce in a large measuring cup or bowl.
Scoop a bit of the chili mixture in the bottom of a 9x9” pan. Place a tortilla on top, then top with a few more spoons of chili mixture. Sprinkle with cheese. Top with another tortilla, more chili, and more cheese. Continue until you’re out of all the ingredients. (I like to sprinkle extra cheese on top.)
Cover and refrigerate at least 3 hours (you can even do it all day or overnight, the longer the better).
When ready to cook, remove from refrigerator and let sit for about 10 minutes (or while oven is preheating, if you're baking it). Place 3 toothpicks in the top so they stick out. Cover with foil (OVEN only) or parchment paper (the toothpicks are so the paper doesn’t stick to the cheese on top).
Microwave instructions: heat in 5 minute increments, letting it sit 1-2 minutes between each, until melted and hot, about 15-20 minutes.
Baking instructions: bake at 350°F 30-40 minutes or until cheese is melted and casserole is hot all the way through, removing the foil or parchment after 20 minutes.