Kosher salt and freshly ground black pepper to taste
Tender romaine or butter lettuce leaves
Store-bought olive tapenade
Store-bought sun-dried tomato spread
Place the potatoes in a large pot of heavily salted cold water, bring to a boil, simmer and cook the potatoes until easily pierced with a fork but not mushy, 21 to 23 minutes. Transfer the potatoes with a slotted spoon to a plate to cool (don’t drain the water). When they’re cool enough to handle, slice them into ½-inch thick slices.
Set up a large bowl filled with ice water. Gently lower the eggs into the boiling water and cook for 8 minutes. Use a slotted spoon to transfer the eggs to the ice water to cool for about 5 minutes. Peel and slice the eggs.
Heat a grill or grill pan over medium-high heat. Pat the tuna dry with paper towels and season well with salt and pepper. Grill, watching the exposed sides of the tuna steak until the uncooked center of the fish is equal in height to the cooked layer on each side, 2 to 3 minutes per side. Remove to a plate, cool, and slice. Set up a make-your-own-sandwich bar by arranging the sliced tuna on a plate and lettuce, tomato, and avocado on another. Put the aïoli, mustard, tapenade, and sun-dried tomato spread in little condiment bowls. Lightly toast the buns and let people build their own sandwiches.