Allow the shredded cheese to come to room temperature (about 30 minutes)
Melt butter in a large (4-5 qt) dutch oven over medium heat. Add the sliced sausage and cook until it just begins to brown.
Add the onion, green onion and garlic to the pot. Saute over medium heat about 5 minutes.
Add the flour and mustard; stir in well and let this cook about 1 minute.
Stir in the stock and beer; add the frozen potatoes. Bring to a boil and cook uncovered for 10-15 minutes or until potatoes are soft.
Stir in the cream and heat through.
Slowly add the cheddar cheeses, stirring constantly, until cheeses are completely melted.
Stir in the Parmesan cheese and red pepper flakes if using.
Transfer to a 4-5 quart slow cooker, cover and set the heat to "warm/keep warm."
Makes approximately 10 servings.