2 teaspoons sugar
1 tablespoon chopped fresh dill
Cook the vegetables: Bring a large pot of lightly salted water to a boil over medium-high heat, covered. Melt the butter in a very large skillet (not nonstick) over medium-high. Add onion, radicchio, and celery seed to skillet and cook until onion is translucent and radicchio softens, stirring occasionally with a wooden spoon, 8 to 10 minutes. Sprinkle flour over onion and radicchio and stir gently until coated. Push vegetables to the side of the skillet to expose the browned bits (the fond) on the bottom of the pan. Add vinegar and quickly stir up the fond. Reduce heat to medium.
Cook the gnocchi: Before adding the cream to the sauce, add gnocchi to the pot of boiling water. It will cook quickly, in just 2 to 3 minutes. The gnocchi is finished when the pieces float to the surface of the water. Drain gnocchi in a colander and quickly return to the empty hot pot to dry and toast gently, about 30 seconds.
Finish the sauce: While the gnocchi is boiling, slowly stir the heavy cream into the skillet with the vegetables and heat until thickened, about 2 minutes. Do not boil. Add the sauerkraut, sugar, and dill. Stir until well-mixed. After toasting the gnocchi in the hot pot, add the gnocchi directly to the sauce in the skillet and stir gently until coated. Serve immediately. If desired, garnish with additional fresh dill.