Optional: Soft pretzel bites and halved miniature sweet peppers
In a large skillet, cook bacon over medium heat until crisp. Using a slotted spoon, remove to paper towels. In the drippings, saute onion until tender. Stir in flour until blended; cook and stir until thickened.
Reduce heat to low. Add cheese cubes; cook and stir until melted. Stir in sour cream, jalapeno if desired, and bacon; cook and stir just until heated through. Transfer to a fondue pot and keep warm. Serve with bread cubes.
Wear disposable gloves when cutting hot peppers; the oils can burn skin. Avoid touching your face.