Heat a large skillet over medium heat and add a tablespoon of cooking oil.
Add the diced onion to the skillet and sauté until translucent, about 3-4 minutes.
Add the ground beef to the skillet, season with salt and pepper, and cook until browned.
In a large bowl, combine the cooked beef mixture with the tomatoes and shredded cheeses.
Place one tortilla in the skillet and fill it with a generous portion of the beef and cheese mixture, then top with a second tortilla.
Cook for 2-3 minutes on each side until crispy and golden brown.
Remove from the skillet and let sit for a minute before slicing into wedges.
Serve hot, accompanied by salsa or sour cream.