Cooking the Pasta

1. Bring a big pot of salted water to a boil and cook spaghetti until just tender. Reserve about 1 cup of pasta water before draining.

Browning the Meats and Vegetables

1. In a large skillet, cook the chopped bacon until crisp. Remove the bacon and set aside, leaving some fat in the pan.
2. Add the sliced bratwurst to the skillet and brown well.
3. Toss in the chopped onion and mushrooms, cooking until the mushrooms shrink and the onion softens.
4. Add minced garlic towards the end to prevent burning.

Making the Beer Reduction

1. Pour in the beer and scrape the browned bits from the bottom of the pan, letting it simmer until reduced slightly.

Preparing the Sauce

1. Lower the heat and add the butter, then sprinkle in the flour, stirring to create a thick base.
2. Slowly add a splash of reserved pasta water. Start adding the cheese, handful by handful, stirring until melted before adding more.
3. Stir in the sour cream and mustard until smooth. Adjust consistency with pasta water as needed.

Combining Pasta and Sauce

1. Add the drained spaghetti directly into the skillet with the sauce and toss gently using tongs. If the pasta clumps, add a splash of reserved pasta water.
2. Stir in the crispy bacon, then taste for seasoning, adjusting salt and pepper as needed.