Pre-heat oven to 375F. Now Put potatoes in a large pot with cold water. Bring the potatoes to a boil with a pinch of salt. After this, Cook on low heat until the potatoes are tender. It should take about 20 minutes.
In a large skillet, heat olive oil. Mix in the Italian sausage, onion, seasoning, and pepper. Once the sausage is almost browned, break up the pieces with a spatula. Cook for 2-3 more minutes to cook the meat. Cover the meat and allow it to cool. If necessary, drain any oil.
Drain potatoes. For a more fluffy consistency, mash the potatoes or use an electric ricer. Mix in the melted butter, warm milk, and salt. Combine until well combined. Mix in the sausage, onion, garlic, and 1and /2 cups of cheese mixture.
Place the mixture in a casserole dish. Top it with the remaining cheese. Add salt and pepper to taste. Bake at 375F for 20 min. After this, Broil the cheese for 2 to 3 minutes on high, or until it is bubbling and golden brown. Garnish with fresh chopped parsley.