Fresh dill sprigs, optional In large bowl,combine egg,bread crumbs,club soda,2 tbsp. chopped dill,3/4 tsp. salt and 1/4 tsp. pepper;using hands or wooden spoon, blend well.Shape mixture into 1 1/4" balls.In 12" skillet,over medium-high heat,melt 2 tbsp. butter;add meatballs;cook about 12 minutes, turning frequently until browned on all sides.Meanwhile,prepare noodles according to package directions,add green beans to boiling water along with noodles;do not add salt.When meatballs are browned,remove to plate using slotted spoon.To drippings in skillet,add remaining 1 tbsp. butter;melt over medium-high heat.Add mushrooms;cook about 5 minutes, stirring frequently until crisp-tender.Return meatballs to skillet along with 3/4 cup water and remaining 1 tbsp. chopped dill,1/2 tsp. salt and 1/4 tsp. pepper.Increase heat to high;bring to boil.Reduce heat to low;cook,covered,5 minutes until meatballs are cooked through. Stir in sour cream;cook 2 to 3 minutes longer until heated through. To serve:Drain noodles and green beans;arrange on serving platter. Spoon meatball mixture over noodles;toss quickly.Garnish with dill sprigs,if desired.Makes 4 servings.