Prep: Finely chop the celery, red pepper and onion, and wash the lettuce leaves; set aside. Place the tuna in a large mixing bowl and use a fork to separate into flakes.
Mix: Add the vegetables and mayonnaise to the tuna as well as salt and pepper to taste, and mix well to incorporate.
Assemble: Place one slice of lettuce, half the tuna salad and half the sauerkraut on each sandwich, slice in half and serve.
Notes
Sauerkraut: Look for raw sauerkraut with live cultures in the refrigerated section of the grocery store (shelf-stable sauerkraut made with vinegar doesn’t contain any live probiotics.)Meal prep: You can make the tuna salad up to 5 days in advance, store it in an airtight container in the fridge, and then easily assemble your sandwiches in the morning.