You can substitute Swiss or cheddar cheese for the provolone. It's important to use the albacore tuna called for in the recipe; albacore is a firm, white tuna that holds up better than flaked tuna.
When picking lemons, look for ones that are firm with skin that is smooth and bright. They will keep for two to three weeks if stored in a plastic bag in the refrigerator. To get more juice out of your fruit, let the lemon return to room temperature, and then roll it across the counter top while applying pressure with the palm of your hand. You'll get about 3 tablespoons of juice from one lemon. Since you only need 1 tablespoon of juice for this recipe, you can either squeeze just one-half of the lemon (and section the rest for use in drinks), or you could squeeze the entire lemon and store the remaining juice in an airtight container in the refrigerator for up to five days.
Preheat broiler. Combine first 8 ingredients in a medium bowl. Divide evenly among muffin halves; sprinkle with cheese. Place on a baking sheet; broil 5 inches from heat for 4 minutes or until golden brown.