Cooked under the broiler, mayonnaise souffles like magic and takes on a lighter texture with a crisp brown shell. There's no better way to top a tuna melt!
Evenly divide tuna between bread; season with salt and pepper. Top with a few slices of dill pickle and a slice or two of provolone.
Stir together mayonnaise, Dijon, and Parmesan. Spread evenly over sandwiches. Broil until cheese melts and mayo puffs slightly and browns in places, 1 to 2 minutes.