In a large soup pot over medium high heat add the butter and let it melt.
Sprinkle in the flour and stir continuously for about 3-5 minutes. We want to cook out the flour taste.
It will start to turn a light golden brown, this is when you can add the onion and start to splash in the chicken stock.
Slowly add chicken stock a few splashes at a time, stirring in between to let the sauce start to thicken.
Once all the stock has been added, the mixture should be pretty thick and bubbly. Stir in shredded cheese and Boursin cheese and let it melt.
Once the cheese melts in, add the beer 1/2 at a time. Same as with the stock, you want to let it thicken up a bit in between so keep stirring!
Sprinkle in the salt, pepper, and whole grain mustard. Stir to combine.
Add in diced potatoes the pot. The starch from the potatoes will continue to thicken the soup! So don't worry if it looks a little thin after adding the beer.
Let this simmer for 10-15 minutes until the potatoes are tender.
Using a hand held immersion blender blend the soup until about half of the potatoes are pureed. If you don't have a hand held blender, remove about 4 cups of soup and blend in a blender until smooth.
Give it a taste and add salt if needed! If it still tastes bitter from the beer it may need more salt and continue cooking for 5 or so minutes.
Serve with large soft pretzels and sliced green onion!