Season tuna: 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.

Assemble: 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.

Toast: 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.