In a large, deep skillet or pot, add the shells, sausage slices, garlic, fennel seed, Italian seasoning, tomatoes, sauce, water, and milk. Stir to combine.
Turn the heat to medium and bring to a simmer. Cover, reduce heat to medium low.
Simmer for 15-20 minutes until pasta is al dente.
Stir parmesan into pasta until melted, adding additional milk if necessary. Serve hot.