Preheat oven to 400 degrees and lightly grease a baking dish. Season both sides of salmon with salt and pepper, then drizzle with lemon juice. Place salmon in prepared dish and bake for 10-15 minutes until pink and flakey.
While salmon is baking, prepare the balsamic reduction. In a small saucepan bring balsamic vinegar to a boil, then reduce heat and allow to simmer for about 15 minutes until slightly thickened. Remove from heat and transfer to a bowl to cool.
Chop salmon into strips or cubes. In a large bowl toss together the salmon, spring greens, lentils, cherry tomatoes, and mozzarella. Just before serving, drizzle with balsamic reduction and sprinkle with black pepper. Serve immediately.