Melt butter in a large skillet; add onions and sauté until onions are tender. Whisk in flour until smooth.
Add evaporated milk and regular milk cooking over medium heat whisking constantly until mixture is smooth and bubbly. Remove from heat and stir in Parmesan cheese.
Place half of the meatballs on the bottom of a lightly greased slow cooker pot. Cover with one-third of the spaghetti sauce. Top with one half of the spaghetti. Pour half of the white sauce over the spaghetti noodles. Top with ½ of the cheddar cheese. Repeat layers once.
Cover and cook on low heat for 4 hours.
Let stand for 10 minutes before serving.
As with any of my recipes, calorie counts and nutritional information varies greatly depending on which products you choose to use when cooking this dish.