Bring large saucepan of water to a boil. Add lentils and onion. Reduce heat and simmer until lentils are tender, about 45 minutes.
Meanwhile, bring medium pot of lightly salted water to a boil. Add carrot strips and cook 1 minute. Remove with slotted spoon and plunge into cold water. Drain and pat dry. In fine sieve over medium bowl, drain tomatoes; set aside.
Drain lentils and cool. Discard onion. Transfer lentils to medium bowl and add celery, red onion, ginger, lemon juice, soy sauce, and salt and pepper to taste. Mix well and set aside.
In food processor, combine cucumber, vinegar, dill, mustard, and salt to taste. Process until mixture is smooth.
Spoon lentils into mound in center of serving plates. Top each with tomatoes and carrots. Drizzle with cucumber dressing and garnish with sprouts.