Gather and prepare all ingredients.

In a medium bowl, fold together the drained tuna, diced celery, red onion, pickles, mayonnaise, salt, and pepper until combined.

To assemble the tuna melts, lay out the sourdough bread and top each piece with a slice of cheddar cheese. Divide the tuna between four slices of bread, then close with the remaining 4 slices to form the sandwiches.

To cook the tuna melts, melt 1 Tbsp butter in a medium skillet over medium heat. Toast two sandwiches for 3-4 minutes, until golden brown. Then add another tablespoon of butter, carefully flip, and cook for 3 more minutes. Repeat with the remaining sandwiches and butter.