In a large saucepan, melt the butter over medium heat.
Once heated, add the sausages and cook until golden brown on each side. Do not cook through. Once browned, remove to a plate and set aside.
Reduce heat to medium low and add the sliced onions. If needed, add a little more butter. Cook, stirring occasionally, until golden brown and caramelized. Adjust heat between medium and low as needed to caramelize the onions without burning.
Add the garlic and cook just until fragrant, 30 seconds to 1 minute.
Stir in the salt, paprika, and black pepper.
Pour in the beer, scraping any browned pieces that have stuck to the bottom of the pan.
Gently add the sausages back into the pan with the beer. Increase heat to medium.
Once boiling, reduce heat to a simmer and cook until the sausages are completely heated through to at least 160˚F (71˚C) and the beer has reduced, about 15 minutes.
While the sauce is simmering, bring a large pot of salted water to a boil. Add the egg noodles and cook until just tender, al dente. Drain and set aside.
Once the sausages are cooked through, reduce heat to low and transfer the sausages to a cutting board. Allow to cool slightly, then cut the Bratwürste diagonally into 1/2 inch (1.25 centimeter) slices.
Stir the mustard into the beer and caramelized onions in the pan, followed by the heavy cream. Adjust seasonings to taste.
Add the sausage slices and heat through before tossing in the cooked egg noodles and fresh parsley.
Serve immediately.