In a small bowl, mix the drained tuna, mayonnaise, and lemon juice until well combined.
Spread the tuna mixture evenly onto one slice of bread.
Place a slice of cheese on top of the tuna mixture.
Top with the second slice of bread.
Heat a non-stick skillet over medium heat and melt the butter.
Place the sandwich in the skillet and cook until the bread is golden brown and the cheese has melted, about 2-3 minutes per side.
Remove from heat, slice in half, and serve warm.