Heat a skillet to medium heat and fry onion in a bit of oil or butter. When onion is translucent, add mushrooms and cook until mushrooms have released their juice and the onion is a little browned, about 5 minutes or so. Remove and reserve in a bowl when done.
Using the same skillet, turn the heat down to medium-low: butter one side of a tortilla and place the buttered side down on the skillet, add some onion/mushroom mix and shredded cheddar cheese. Place another tortilla on top, but with the buttered side facing towards you. Cook for a couple minutes, until cheese starts to melt and the bottom tortilla is lightly browned. Flip and repeat. Remove from skillet and repeat with remaining ingredients.
Cut in quarters and serve. It's good on it's own, or with typical condiments like sour cream and salsa.