In a large nonstick skillet placed over high heat, pour 2 teaspoons oil. Heat until hot and shimmering.
Add scallops in a single, nontouching layer, Cook until bottoms are browned, about 2 minutes. Flip and cook until browned on both sides and scallops are just cooked through, about 2 to 3 minutes more. Remove to a plate.
Heat remaining teaspoon of oil in same skillet.
Add onion. Stir fry until lightly browned, about 2 minutes.
Add mushrooms. Stir fry until mushrooms and onion are almost tender, about 2 minutes.
Stir in ginger and garlic. Stir fry until fragrant, about 30 seconds.
Stir in 1 cup of carrots, bean sprouts and any juices that have accumulated on scallop plate. Stir-fry until carrots and sprouts have softened slightly, about 1 minute. Remove from heat.
Stir in soy sauce and lemon juice.