Set your largest pan (cast iron best) over high heat. As it heats, slice your onions as thinly as you can (using a mandolin wise here). Once sliced, add the butter to the pan to melt.
Once butter is melted, add in the onions and let sit untouched for 4 minutes. Then, add in a few ounces of beer, and stir (get those burnt bits off of the bottom of the pan).
Repeat this process a total of 5 times or for 20 minutes. Doesn't have to be exact, but the goal is to not have much liquid left in the pan at the end.
Once the beer is reduced and the onions browned, lower the heat and add the salt/garlic/pepper. Then, turn off the heat and stir in the mustard. Serve right away or chill for 2-3 days (not that they will last that long).