Grill or pan fry brats until completely cooked. Cover and set aside.
Cook pasta according to package instructions. While pasta cooks, heat oil and butter in a medium pot or a large rimmed skillet over medium heat. Add onion and saute till soft. Stir in flour. Gradually whisk in the milk and allow the sauce to bubble for about 4 minutes stirring often.
Remove pasta from heat, drain and set aside.
Whisk the beer into the sauce, and allow it to cook for and additional 3-4 minutes stirring often.
Fold in the cheese and stir till completely melted. Gently stir in the pasta. Season with salt and pepper.
Slice brats and stir into pasta. Serve.