Use a fork to break up the tuna into smaller flakes. This will mainly be needed if using solid white tuna. Combine the fish with the mayonnaise, celery, red and green onions, relish, dijon mustard, parsley, lemon juice, salt, and pepper.
Mix until the tuna mixture is combined. It yields about 2 cups, ½ cup per sandwich. The salad can be prepared up to 5 days in advance, making it ideal for meal prep.
Butter the bread
The fat and milk solids in butter help the bread develop a golden brown hue and flavorful surface. Use softened butter to make it easy to spread a thin layer on one side or each piece. I also find it easy to brush on melted butter for an even coating, especially if the bread tends to be more delicate.
You can skip this step. However, the bread will be drier. If you want to reduce the amount of butter, you can lightly grease the pan each time the sandwich is flipped. Vegetable oil can also be used. I’ve also used mayonnaise to spread onto the bread for a crisp surface, but it will have a slightly eggy flavor.
Build the sandwich
To ensure that the cheese starts to melt first, it’s important to layer the sandwich a certain way. Add the cheese slices or grated cheese to the bread so it’s on the bottom. This gives it closer contact with the hot pan. Add the tomatoes on top, followed by ½ cup of tuna salad.
Place the other piece of bread on top. Now you’re ready to toast the sandwich! Alternatively, add cheese to the top and bottom for extra gooeyness. This also helps the sandwich stick together if you add additional ingredients to the tuna salad.
Cook the sandwich
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