Tuna's resemblance to beef is really emphasized when you make hamburgers out of it; like beef burgers, tuna burgers should be cooked rare to medium-rare. Unlike beef burgers -- which have a wonderful, intrinsic flavor -- the relatively tasteless tuna burgers are great when piqued by strong flavorings. This flavorful dish goes in a Mediterranean direction. It's delicious with aioli -- or another mayonnaises (such as lemon or saffron). Makes 4 tuna burgers.
Trim tuna, removing skin, gristle, and any dark meat. Chop by hand until tuna is the texture of coarse hamburger meat. Chop it as evenly as possible. Place in a large mixing bowl.
Combine mustard, thyme, ground fennel, cayenne, and salt in a small mixing bowl. Stir well to combine. Add to the chopped tuna and fold together carefully, making sure the mustard is evenly distributed. Divide tuna into 4 balls, and gently flatten into 1-inch-thick patties.
Heat a large skillet over moderately high heat. Coat bottom of skillet with a thin film of olive oil. When oil is hot, cook patties about 2 minutes per side, until nice and golden on the outside and rare to medium-rare on the inside.
Serve on toasted sesame rolls or English muffins, slathered with aioli.