Step 1In a medium bowl, use a fork to break up the tuna into small pieces. Stir in one-fourth cup mayonnaise. Rinse the capers in a strainer under cold running water several times to remove excess salt; drain and pat dry on paper towels. Chop the capers if they're large; otherwise, leave them whole. Stir them into the tuna salad, along with the lemon juice and freshly ground black pepper to taste.
Step 2Spread 1 teaspoon mayonnaise over the top half of each muffin. Spread one-fourth of the tuna salad on the bottom half of each muffin, and top each mound of salad with 1 tablespoon of the shiso leaves.
Step 3Rinse the onion slices in a colander under cold running water twice, then drain and pat the slices with paper towels. Divide the onion, romaine and tomato slices evenly among the four sandwiches, on top of the shiso and tuna salad. Top each sandwich with a second muffin half. Cut in half and serve immediately.