If you want to add more protein to your tuna salad sandwich, pair it with a slice of cheese. Several options can enhance the flavors of this classic sandwich. One popular choice is Swiss cheese, which has a nutty and slightly sweet flavor that blends well with tuna. Another great option is cheddar cheese, which adds a sharp, tangy taste and a creamy texture to the sandwich. For those who prefer a more mellow and creamy taste, Havarti cheese can be a great option. Ultimately, the choice of cheese comes down to personal preference, and experimenting with different flavors and combinations can give you an exciting twist on a classic tuna salad sandwich.