Cook macaroni al dente, according to package instructions. Drain, rinse and immediately add pasta back to same pot. Over low heat, immediately add 1 cup cheddar cheese, 1 tablespoon butter, salt, green onions and heavy cream. Mix thoroughly until cheese melts and coats macaroni. Turn off heat and set aside.
Butter 2 bread slices on both sides and set aside. In cast iron or nonstick skillet, heat 1 tablespoon butter over medium heat. When almost completely melted, place the 2 slices of buttered bread in skillet and top with 1/4 cup of grated cheddar cheese. Cover until cheese is almost entirely melted, about 2-3 minutes.
Remove lid and top melted cheese with prepared macaroni and cheese; re-cover skillet for a minute or 2. Butter one side of remaining 2 bread slices and spread Dijon mustard on the other side. Remove lid from skillet and place bratwurst slices over macaroni and cheese and top with sauerkraut. Place Dijon mustard-side-down slices over sauerkraut and carefully flip sandwich to toast this side, adding more butter if necessary. Do not cover skillet. Flip sandwich one more time, if desired, to crisp toast.