Step 1: Drain and Prepare the Tuna
Open your cans of tuna and drain them thoroughly by pressing down with a fork or using a small strainer. You want to remove as much liquid as possible—excess moisture will make your sandwich soggy. Transfer the drained tuna to a medium mixing bowl and use a fork to flake it into smaller, even pieces. This creates better texture and helps the mayonnaise coat every bite.

Step 2: Prepare Your Vegetables
While your tuna is draining, dice your celery and red onion into very small, uniform pieces—about ¼-inch or smaller. Small pieces distribute better throughout the mixture and provide crunch without overwhelming bites. If you’re sensitive to raw onion, you can rinse the diced pieces under cold water to mellow the flavor.

Step 3: Make the Tuna Salad
Add the mayonnaise to your flaked tuna and mix well with a fork until evenly combined and creamy. Add the diced celery, red onion, lemon juice, Dijon mustard (if using), garlic powder, black pepper, and salt. Fold everything together gently but thoroughly until all ingredients are well distributed. Taste and adjust seasoning—you might want more lemon juice for brightness, more mayo for creaminess, or more pepper for spice.

Step 4: Let It Rest (Optional but Recommended)
If you have time, cover the bowl and refrigerate the tuna salad for 15-30 minutes. This resting period allows the flavors to meld together beautifully and makes the mixture easier to spread. However, if you’re in a hurry, you can skip this step and proceed immediately.

Step 5: Assemble Your Sandwiches
Lay out your bread slices on a clean surface. If you’re toasting your bread, do that first and let it cool slightly. Place lettuce leaves on the bottom slice of each sandwich—this creates a barrier that prevents the bread from getting soggy. Divide the tuna salad evenly among your sandwiches, spreading it generously from edge to edge. Add tomato slices and cheese if desired. Top with the second slice of bread and press down gently.

Step 6: Cut and Serve
Cut each sandwich in half diagonally (or straight down the middle—your preference!). Serve immediately, or wrap tightly in plastic wrap or parchment paper if packing for later. Enjoy with your favorite chips, pickles, or fresh fruit on the side!