Drain and rinse 4 cans of pinto beans.
Add some olive oil to a large sauce pot on medium-high heat, and sauté chopped onion for 6 minutes. Sprinkle with salt and pepper. Stir in minced garlic and cook for another minute.
Add in your beans, 2 cups of chicken stock and ½ teaspoon both salt and pepper. Put the lid on and let it come up to a boil, then turn it down to medium-low for about 30 minutes.
Take off the lid and bring it back up to medium or medium-high to cook off some of the liquid, for about 10 minutes. Stir regularly so the beans don’t burn on the bottom.
Turn off the heat, and let the beans cool about 5 minutes. Then mash the beans with a potato masher.
Add half of the evaporated milk and continue mashing until the beans are creamy. Toss in the cheddar cheese, do not stir and let the beans rest for 5 minutes. Add in the rest of the evaporated milk and stir gently. Ideally leave some of the melted cheese in very small chunks or ribbons. Serve warm.
Heat tostadas in a toaster oven for one and a half minutes, or on a cookie sheet in a 350 degree oven for one and a half minutes.
To assemble, start with a layer of refried beans.
Next layer on meat of your choice, cheese, sour cream, onions, jalapenos, and cilantro.
For a vegetarian pizza, omit the meat.
For huevos rancheros, replace the meat with fried or scrambled egg.