How to Make It
Cook pasta according to package directions; drain and keep warm, reserving 1/2 cup of cooking water.
Heat oil in a large skillet over medium-high. Add sausage and onions; cook, stirring occasionally, until sausage is browned and onions are slightly caramelized. Deglaze skillet with beer, and cook until liquid is reduced to about 1/4 cup. Remove from heat, and keep warm.
Heat butter in a medium saucepan over medium. Whisk in flour, and cook, whisking constantly, 1 to 2 minutes. Whisk in milk, and bring to a simmer. Reduce heat to medium-low, and cook, whisking often, 5 minutes.
Stir cheese and mustard into milk mixture; remove from heat. Add apple cider vinegar, salt, and pepper; stir to combine.
Stir sausage mixture and sour cream into cheese mixture; pour over pasta, and toss to coat. Stir in cooking water if mixture seems too thick. Toss with fresh herbs, and serve immediately.