Preheat oven to 350 degrees F (175 degrees C). Lightly grease one 3 quart casserole dish.
Bring a medium saucepan of salted water to a boil. Stir in onions; cook until tender but still firm. Drain.
Melt the butter in a large skillet over medium heat. Stir in the flour and continue stirring until golden brown. Pour in the milk and allow the mixture to thicken. Mix in the cheese, allowing it to melt.
Arrange onions in casserole dish and cover with cheese mixture; sprinkle with paprika.
Bake in the preheated oven 30 minutes, or until heated through and bubbling around edges.
