Boil large pot of water for the pasta. While you wait for the water to boil, brown the Karl Ehmer Bratwurst, about 1-2 minutes on each side, then set aside.
Boil the pasta according to package directions. One minute before pasta is ready, add peas to boiling water to blanch with the pasta. Drain pasta and peas, set aside.
In pot you cooked the pasta, melt butter over medium heat and add contents of cheese packet, milk, shredded cheddar, and brown mustard. Stir to combine cheese sauce. When cheese sauce is warm and combined, add pasta and peas as well as the Karl Ehmer bratwurst to pot. Stir to combine.
You can serve immediately straight out of the pot or you can transfer meal from pot to a 9-inch square or large pie pan baking dish dish and cook in preheated oven for 5-10 minutes. If making ahead, cover with aluminum foil and heat in oven until heated through, about 20-30 minutes, before removing foil. Then cook for another 5 minutes uncovered until bubbling and golden.