Heat oven to 425°F; line a sheet with parchment. Mix beef, grated onion, egg, panko, and salt; shape twenty equal meatballs on sheet.
Boil rice and 1 3/4 cups water in a saucepan. Cover; cook on low 18 minutes. Rest covered off heat 10 minutes.
Bake meatballs 14 to 17 minutes until browned and centers reach 160°F.
Combine soy sauce, mirin, sake, sugar, and ginger in a small saucepan. Simmer uncovered 8 to 12 minutes until reduced to about 1/2 cup and lightly syrupy; watch closely near end.
Heat oil in a skillet over medium-high. Add carrots, snow peas, and 3 tablespoons water; cover 2 minutes. Uncover; cook 1 minute until water evaporates.
Toss meatballs with sauce. Divide rice and vegetables among four bowls; add five meatballs each and spoon over remaining glaze.