Step 1: Thoroughly drain the canned tuna. Press down on the tuna in a colander to remove as much liquid as possible. Transfer the drained tuna to a medium mixing bowl and flake it apart with a fork.
Step 2: Add the finely minced onion, beaten egg, breadcrumbs, mayonnaise, Dijon mustard, chopped parsley (if using), salt, black pepper, and cayenne pepper (if using) to the bowl with the tuna.
Step 3: Gently mix all the ingredients together until just combined. Be careful not to overmix, as this can make the patties tough. The mixture should hold together when pressed.
Step 4: Divide the tuna mixture into 4 equal portions. Shape each portion into a patty, about 1-inch thick and 3-4 inches in diameter. If the mixture is sticky, lightly moisten your hands with water.
Step 5: Heat 2-3 tablespoons of olive oil or vegetable oil in a large non-stick skillet over medium heat until shimmering. Carefully place the tuna patties into the hot skillet, ensuring not to overcrowd the pan. Cook in batches if necessary.
Step 6: Cook the patties for 3-5 minutes per side, or until they are golden brown and crispy on both sides and heated through. The exact time may vary based on your stove and patty thickness.
Step 7: Once cooked, transfer the crispy tuna patties to a plate lined with paper towels to drain any excess oil.
Step 8: Serve immediately with your favorite dipping sauce, such as tartar sauce or lemon-garlic aioli, or alongside a fresh salad or sandwich bun. Enjoy your homemade classic crispy tuna patties!