Preheat oven to 375 degrees. Grease a 13x9 pan, and set aside. This part is easy peasy...chop smoked sausge in chunks, like 1 1/2 inch pieces, chop potatoes in chunks, mine were like 1/2 inch slices, and chop head of cabbage in chunks, thick chunks are great. Place in greased 13x9 in pan.
In a large sauce pan on medium heat, melt butter and blend in flour to create a roux, stir with a whisk. Slowly add in milk, continuing to stir with a whisk. Add salt, onion salt, paprika, cayenne pepper, black pepper and garlic. Stir constantly, sauce will thicken, usually about 7-10 minutes of stirring. Sprinkle cheddar cheese into sauce mixture, continue to stir until melted completely. Sauce will be somewhat salty, but this will bake into potatoes and cabbage for a perfect blend.
Pour cheese sauce directly onto cabbage, potatoes and smoked sausage. Stir. Cover with aluminum foil and bake for 45 minutes at 375 degrees. Un-cover foil and continue to bake for an additional 10 minutes. Let stand for 5 minutes, serve.