Preheat the oven to 350. Grease a 9 x 13-inch baking pan with nonstick cooking spray.
Soften the tortillas by heating them in the microwave or in a hot dry skillet, or by frying them in an inch of hot oil until softened and a bit leathery. If frying, drain them on paper towels before proceeding.
In a large skillet, cook the sausage and onions, breaking up the sausage into smaller bits, until the sausage is cooked through and the onion is translucent.
Divide the sausage mixture among the tortillas and top with a few pinches of cheese, reserving at least one cup of cheese for the top. Roll the tortillas up, placing each rolled enchilada, seam side down, in the prepared pan.
Pour the enchilada sauce over the top. Sprinkle the remaining cheese over the top. At this point the enchiladas can be stored in the refrigerator until ready to bake. If desired, you can freeze the chilled enchiladas, covered, for up to 2 months. Thaw in the refrigerator before baking.
Bake until hot and bubbly, about 15 minutes.
If baking a cold casserole, add 5-10 minutes to the baking time. Sprinkle cilantro over the top before serving.