Cut beef into thin slices, then into 1-inch squares. Heat 1 Tbsp. oil in large skillet over high heat. Add beef and stir-fry 1 minute; remove. Heat remaining 1 Tbsp. oil in same pan. Add tomatoes, mushrooms, white parts of green onions and garlic; stir-fry 1 minute. Stir in soy sauce and 2 Tbsp. water; reduce heat and simmer, covered, 5 minutes. Combine sweet & sour sauce, brown sugar and ginger; stir into tomato mixture with green onion tops. Cover and cook 1 minute. Stir in beef. Serve over hot capellini.