Heat 2 teaspoons oil in a large nonstick skillet over medium-high heat. Add mushrooms, shallots, ginger, and garlic; sauté for 3 minutes or until mushrooms are tender. Add sherry, 5 teaspoons soy sauce, and sugar; cook for 1 minute or until liquid is almost absorbed. Remove from heat, and stir in onions. Spoon mushroom mixture into a medium bowl; keep warm.
Return pan to medium-high heat; add remaining 1 teaspoon oil. Add one-third of spinach, and cook 2 minutes or until spinach wilts, stirring frequently. Repeat procedure in 2 more batches with remaining spinach. Spoon spinach into a medium bowl; stir in remaining 2 teaspoons soy sauce and lemon juice. Top with mushroom mixture.