Prep the tuna: Drain tuna very well and flake it with a fork.
Excess moisture makes patties fall apart, so press the tuna lightly with the lid to remove liquid.
Mix the base: In a bowl, whisk the egg, mayo, Dijon, lemon zest, salt, pepper, and minced garlic until smooth.
Add dry ingredients: Stir in Parmesan, breadcrumbs, parsley, and onion. Mix to a moist but scoopable consistency.
Fold in tuna: Gently combine the flaked tuna with the mixture. Don’t overwork it.
If it’s too wet, add a tablespoon more breadcrumbs; if too dry, a teaspoon more mayo.
Shape patties: Form 6–8 small patties (about 1/3-inch thick). Press edges so they hold together. Chill for 10–15 minutes if you have time; it helps them set.
Heat the pan: Set a large skillet over medium heat.
Add enough oil to coat the bottom. Wait until the oil shimmers.
Cook: Place patties in a single layer without crowding. Cook 3–4 minutes per side until golden brown and crisp.
Adjust heat so they brown without burning.
Finish and serve: Transfer to a paper towel–lined plate and sprinkle with a tiny pinch of salt. Serve with lemon wedges, a simple salad, or in buns with lettuce and tomato.