Place the eggs in a pot and cover with water. Bring to a boil over medium-high heat. Cook for 8 minutes. Drain and cool under cold running water, then peel.

Chop the peeled eggs and transfer to a bowl. Add mayonnaise, Dijon mustard (if using), salt, and black pepper. Mix until well combined to form the egg salad.

Toast the sourdough bread until lightly crisp.

Layer the toasted bread with spinach, tomato, egg salad, and red onion. Close the sandwiches.

Combine sparkling water, lemon slices, fresh mint. Add ice cubes if desired and stir gently.

Serve the egg salad sandwich with apple slices and cheese alongside the mint lemon sparkling water.