Preheat oven to 375°F and set out a 15-inch oven-safe skillet.
In a medium saucepan, melt the butter and diced onion over low heat until the onion is softened (3-5 minutes). Then, add the flour, salt, and pepper and cook for 1 minute, stirring constantly.
Slowly drizzle in milk, whisking constantly. Continue cooking until the mixture thickens (about 5 minutes), whisking frequently.
Stir in the cheese, and continue stirring until melted and smooth. Set aside.
To the skillet, add the potatoes, fanning them out into a single layer. Stagger the slices of sausage in between the potato rounds.
Pour the cheese sauce over the top of the potatoes and sausage.
Cover the skillet with aluminum foil and bake for 60 minutes, or until potatoes are fork-tender.
After 60 minutes, remove the foil and broil for 4-5 minutes, watching carefully, to brown the top.
Serve hot, garnished with parsley. Salt and pepper to taste.