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.