Brown the sausage in a frying pan over medium-high heat, stirring often. Cook about 7 minutes, until browned and crumbly. Add the shallots and sauté together 2 minutes, then drain off any excess fat and set aside.
Stir together the mashed potatoes, milk, and poultry seasoning until smooth. Add the sausage and mix until well combined.
Spread the potato mixture in a baking dish, and top with the melted cheese. Bake for about 20-30 minutes, until the cheese is bubbly and the casserole is warmed through. Top with the fried onions and bake 5 more minutes, then serve.