Boil pasta according to package directions, drain, and set aside.
Preheat oven to 400°
Meanwhile, a large pan over medium heat, brown ground beef and chopped onion, stirring occasionally and breaking up the meat into small pieces. Cook the ground beef through, 10 - 15 minutes.
Drain the grease from the pan.
Add tomato sauce, water, and salt & pepper to taste. Bring to a simmer and cook, stirring occasionally for 20 minutes. Remove from heat.
In a separate small sauce pan, make your béchamel by melting the butter, and whisking in the flour. Then slowly add in the milk (about 1/2 cup at a time) whisking frequently. When the sauce gets thick, add the next 1/2 cup of milk and repeat the process until all the milk is added in. Add cheese and stir to combine.
In a large (at least 4 quarts) casserole dish, layer half of the cooked pasta, then 5 slices of American cheese, then add the meat sauce, the remaining pasta, and another layer of 5 slices of American cheese. Top with béchamel sauce.
Cook in oven for 30 minutes, until top is golden brown.
