Heat a large stockpot over medium high heat and add the bacon; cook until crisp, and then remove with a slotted spoon, set aside to cool, and crumble. Add the bratwurst to the pot and cook in the bacon fat until lightly browned. Remove and set aside. Drain off excess fat.
Add the onion to the stockpot and cook until slightly softened, about 3 minutes. Deglaze the pan with the beer over high heat, using a wooden spoon to pick up any browned bits from the bottom of the pan. Continue to cook for 3 minutes to cook off the alcohol and reduce a bit.
Add the crumbled bacon, garlic, thyme, potatoes, stock, and pepper and bring to a boil. Cover, reduce heat to simmer, and cook until the potatoes are very soft, about 20-30 minutes.
Crush the potatoes with a potato masher, or lightly puree with a stick blender, taking care to leave the soup somewhat chunky in texture. Return the bratwurst to the pot and heat through for 5 minutes.
Ladle into soup bowls and garnish with parsley to serve.
