Mix tuna, mustard, pickles, salt, & pepper together until well combined.
Place slice of swiss on top of tuna mixture and microwave on high for 30 secs or until cheese is melted over the top.
Toast the bun, add a bit more mustard and the spinach leaves to the base of the toasted bread. Remove tuna melt from microwave and carefully slide on the toasted bun. Top with remaining thin sandwich bread slice.