Step 1

Add the mayonnaise, lemon juice, and mustard to a mixing bowl, and stir well.

Step 2

In another bowl, combine the tuna, sweet relish, and onion.

Step 3

Pour the mayonnaise mixture over the tuna mixture and stir to coat.

Step 4

Season with salt and pepper.

Step 5

Serve on bread slices and garnish with sliced tomatoes.