Place the bacon on a baking sheet lined with foil and bake until lightly brown but not completely crisp, 15-20 minutes. Remove to a paper towel-lined plate and keep warm (I put an upside down plate over the top of the bacon).
For the beer cheese sauce
Melt the butter in a medium saucepan over medium heat.
Add the onions and cook until soft, 2-3 minutes.
Stir in the flour and cook, stirring, until it starts to brown, about 2 minutes.
Whisk in the beer until smooth.
Reduce the heat to low and simmer until the beer is cooked out and the sauce is reduced, about 5 minutes.
Stir in the cream and garlic. Stir continuously and cook until the sauce has thickened, 10 minutes.
Remove from heat and season with 1/2 teaspoon salt and a pinch of pepper.
Slowly add the cheeses and stir until melted.
Keep warm over low heat.
For the burgers
Cut butter into small pats.
Form beef into 4 balls, Press butter into centers then form into patties Season with salt and pepper.
Heat a grill or grill pan over medium high heat.
Add patties and cook on one side until browned.
Flip and cook until done.
Top with cheese slices (see my note below) and remove to a plate.
Butter both sides of the pretzel buns and place on the grill, cut-side down. Grill until golden brown and toasted.
For the burgers
Top buns with cheese, bacon, burgers, and cheese sauce. Serve with plenty of napkins!
Note: I like to put the cheese on my bottom bun instead of on the grilled patties. It helps form a barrier between the juicy meat and the bun, preventing it from getting soggy.