Melt butter in a medium saucepan over medium-high heat. Add onion, and sauté until the onion begins to soften.
Add sausage until it is cooked through, about 8-10 minutes.
Drain off any fat from the sausage and add the mustard, cream cheese, tabasco, and beer to the pan. Stir until the cream cheese is melted then stir in the cheddar cheese.
Transfer mixture to a fondue pot and serve with pretzel sticks.