In a medium bowl, combine tuna, mayonnaise, onion, lemon juice, and oregano and stir until thoroughly combined. Add more mayo if you want mixture wetter. Season with salt and pepper, to taste.
Butter outsides of bread slices. Layer tuna mixture on inside portion of bread. Add cheese, desired toppings, then another piece of cheese and top with bread.
Heat a skillet over medium-low heat. Add sandwiches and cook until bottom bread is toasted, and cheese is warm. Flip and cook on the other side until bread is toasted and cheese has melted.
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Notes
Tuna: Choose high quality canned tuna. White Albacore tuna, packed in water, is my preferred choice for tuna melts. Be sure to drain really well before making the filling. Cheese: Use your favorite kind of cheese--pepper jack, American, or any kind will work!Bread:Wheat, white, sourdough, rye, beer bread, or even crusty artisan bread.Toppings: Keep it simple with tuna and cheese or jazz it up by adding tomato, avocado, bacon, sprouts, jalapeños or hot sauce for a spicy tuna melt.Open Face: Leave out the top layer of bread and broil in the oven as an open-face tuna melt.Air Fryer Tuna Melt: Air fry at 370 for 2-3 minutes on each side, until the cheese is melted.Tuna Melt Wrap: Add tuna mixture and cheese to a tortilla and fold like a burrito. Cook on a skillet over medium heat until toasted on all sides and the cheese is melted.Make Ahead Instructions: The tuna filling can be made ahead of time and kept in the fridge. This makes for an even quicker lunch!Storage Instructions: Keep the tuna mixture in an airtight container in the fridge for up to 4 days.