Peel the beetroot and cut into thin slices. Cut the onion into thin rings and sauté in the butter. Add beetroot and sauté. Salt and pepper. Pour in the stock and cook for 15 minutes.
Cook the pasta according to the instructions on the packet. Chop the dill. Drain the pasta and mix with the beetroot. Let simmer for approx. 1 min. Season to taste with vinegar, salt and pepper. Sprinkle with horseradish and dill.