Brown ground beef. Combine with sour cream, onions, and half of cheese.
Soften tortillas. For each tortilla, spoon a 1-inch column of sour cream mixture down the middle. Roll up tortilla around filling and place in baking dish sprayed with non-stick spray. (I find it helpful to tear off about 1/4 of each tortilla before rolling around mix, so it ends up softer and less bready).
Pour sauce over enchiladas, being sure to spread over all exposed tortilla. Sprinkle remaining cheese on top.
Bake uncovered at 350°F for 15-20 minutes, until bubbly and cheese is melted. I serve with sour cream and salsa on the side.