- In medium saucepan, melt the butter. Remove from heat; blend in the sour cream and flour to make a paste. Whisk in the milk and half-and-half and bring to a boil over medium heat, stirring frequently.
- Reduce heat to simmer, stir in salt, pepper, and cayenne, and stir frequently until the sauce thickens (about 5 minutes).
- Whisk in an additional tablespoon of flour, if needed, for desired thickness.
- Remove from heat, stir in shredded cheeses and stir well. Pour cheese into fondue pot or slow cooker set on high.
- Serve with cubes of sourdough or French bread, or lightly cooked vegetables (broccoli or cauliflower florets work well).