Preheat oven to 350 degrees.
Heat about half an inch of oil in a heavy skillet (the oil is ready when it gently bubbles around the submerged tortilla).
Pass each tortilla through the oil, flipping once, to soften, just a few seconds. Place tortillas on a paper towel–lined baking sheet.
Spoon a little sauce into the bottom of an oven-proof dish.
Top each tortilla with a row of cheese and onion, roll up, and place seam side down in the dish.
Ladle more sauce on top of the enchiladas, then sprinkle with more cheese and onion.
Bake uncovered for 20 minutes or so, until the cheese is bubbly.