Slice onions into thick rings.
In a medium-sized pan, sauté onions in butter until softened, about 5 minutes. Remove onions from pan and place into the crockpot. Set pan aside, you will use it again.
In a mixing bowl, combine Dijon mustard, ketchup, vinegar, brown sugar, honey, Worcestershire, lemon juice, paprika, garlic powder, salt, pepper, and hot sauce. Mix well and set aside.
In the same pan you softened the onions in, brown brats over medium heat for 3 minutes on each side (brats will not be fully cooked).
Add the sauce and brats to the crockpot (on top of the onions) and stir to combine.
Pour beer over the top and cover.
Turn the crockpot on low and cook on low 6 to 7 hours.