In a nonstick frying pan, heat the water over medium heat. Add the shrimp and onion, and saute until the shrimp is opaque and the onions are translucent, 5 to 7 minutes. Transfer to a bowl, cover and refrigerate until well chilled.
In a small bowl, add the apples and lemon juice. Toss to coat evenly and set aside.
In a large bowl, combine the celery, parsley, dill, horseradish and mayonnaise. Add the black pepper to taste. Stir in the shrimp mixture and the apples. Refrigerate until well chilled, 45 to 60 minutes.
Just before serving, stuff the shrimp salad into the cored tomatoes. Serve immediately.