Step 1
Place the tuna in a bowl and add mayonnaise, mustard, onion, and pepper. Mix everything well until the filling becomes smooth and evenly combined.
Step 2
Lay out the rye bread slices on a flat surface. Spread the tuna mixture evenly over two slices, making sure the filling reaches the edges for balanced flavor in every bite.
Step 3
Place two slices of cheese over the tuna mixture on each sandwich. Cover with the remaining rye bread slices to form two complete sandwiches.
Step 4
Spread butter on the outer sides of each sandwich. Heat a skillet over medium heat and place the sandwiches in the pan carefully.
Step 5
Cook each sandwich for 3 to 4 minutes on one side.
Step 6
Flip gently and cook the other side until golden brown.
Step 7
Continue until the cheese melts fully and the bread turns crisp.
Step 8
Remove the sandwiches from the skillet and let them rest for a minute. Slice in half and serve warm.
