Heat oil in a large skillet. Add sausage and break up in skillet to cook. Once sausage is 3/4 done, add mushrooms (if using) and garlic. Drain excess oil from skillet and add Alfredo Sauce. Pour the milk into the Alfredo sauce jar, and add to mixture. Heat through. Mixture will thicken up as it cooks.
Meanwhile, cook tortellini as package instructs. Turn on your broiler when the pasta is done.
Combine pasta and sauce mix, and pour into either a large round casserole or a 9x13 cake pan. Top with as much cheese as it takes to cover the majority. Place under your broiler until the cheese just starts to bubble and turn brown. Let this cool off for about 10 minutes, and then dig in.