Steps

1. Finely mince 1/4 onion. (Key Tip! It's easier to mince if you keep the root end attached. If it seems too large, press down on the tip of the knife and gently tap to mince it finer.)
   🕒 01:30
2. Transfer the minced onion to a bowl.
   🕒 04:09
3. Drain the oil thoroughly from 1 can of tuna and add it to the bowl with the onion.
   🕒 04:32
4. Add 2 tsp mayonnaise, 1/2 tsp lemon juice, 1/2 tsp potato starch, and black pepper to taste to the bowl and mix well. (Key Tip! Adding potato starch prevents the onion from releasing moisture and becoming mushy when grilled. The starch's moisture-retaining effect results in a creamy, gratin-like texture.)
   🕒 05:21
5. Lightly wet 2 sheets of parchment paper, crumple them, and then wring out the excess water. (Key Tip! Wetting and crumpling the paper makes it softer and easier to wrap the filling.)
   🕒 06:37
6. Cut 2 slices of bread in half. (Key Tip! The main reason for cutting the bread in half is for easier eating. It also prevents the bread from collapsing under the weight of the filling.)
   🕒 07:44
7. Place the cut bread slices on the parchment paper and spread a thin layer of extra mayonnaise on the surface. (Key Tip! The oil in the mayonnaise acts as a barrier between the bread and the filling, preventing moisture from soaking into the bread.)
   🕒 08:25
8. If desired, spread a small amount of mustard thinly on one side of the bread. (Key Tip! A small amount of mustard adds a nice accent and enhances the flavor.)
   🕒 09:26
9. Layer 2 slices of melting cheese, torn in half, on each piece of bread.
   🕒 10:10
10. Generously top the cheese with half of the prepared tuna mixture on each.
    🕒 10:54
11. Cover with the other slice of bread, then firmly wrap the sandwich with the parchment paper, pressing down to prevent the filling from leaking, and fold the edges securely. (Key Tip! The parchment paper acts like a burger wrapper, keeping your hands clean and making it easy to eat even if the filling leaks. It also reduces the amount of dishes to wash.)
    🕒 12:21
12. Place the wrapped sandwich, folded side down, in a cold frying pan. Place a weight on top and cook over medium-low heat for 3 minutes. (Key Tip! Grilling the folded side first helps to secure the parchment paper and makes it less likely to peel off. Pressing with a weight creates a crispy exterior and a gooey interior.)
    🕒 13:16
13. Flip the sandwich, place the weight back on top, and cook over medium-low heat for another 3 minutes. (Key Tip! Don't worry if the bread looks a little browned; the paper prevents it from burning too much. A slightly charred appearance actually enhances the flavor, so cook until both sides are golden brown.)
    🕒 15:05
14. Remove the weight, flip again, and place the first-grilled side down. Cook for 2-3 minutes to make the surface crispy. (Key Tip! By not using a weight in the final step, moisture doesn't get trapped, preserving the crispy texture.)
    🕒 15:52
15. Once nicely browned, it's ready! Enjoy the crispy texture right away while it's fresh.
    🕒 17:17