Preheat the oven to the "keep warm" setting (170°F).
Drain the tuna thoroughly. It needs to be very dry, as shown in the photo. If too much moisture remains, the patties will fall apart. Place the drained tuna in a medium bowl and flake it with a fork. Make sure no big chunks of tuna remain. It needs to be well-flaked.
Mix in the mayonnaise, Dijon mustard, minced garlic, eggs, kosher salt, black pepper, and parsley.
Heat the olive oil in a large 12-inch nonstick skillet over medium heat.
Using a 4-tablespoon scoop or measuring cup, transfer four mounds of the tuna mixture to the hot oil. Pack the mixture into the scoop - this will help the patties retain their shape.
Gently flatten the mounds with the back of a fork.
Cook the patties until golden brown, about 3 minutes per side. Don't try to flip them too soon - they should form a brown crust on the bottom before you flip them. Keep the cooked patties in the warm oven while you fry the second batch.
Serve immediately.
Video
Notes
The eggs should be truly large. If they are on the small side, add an extra egg yolk (or egg white).
You will use 4 tablespoons of oil for frying, but the tuna patties will only absorb about half that amount. The nutrition info reflects that.
Different canned tuna products have different textures. It's important to drain the tuna so that it's visibly dry - place it on paper towels to drain if needed. If, despite your efforts, you mix the ingredients and the mixture seems very liquid, add 2-4 tablespoons of breadcrumbs (regular or gluten-free) or (for a low-carb, grain-free version) almond flour.
Dijon mustard is high in sodium. It typically has 120 mg of sodium per teaspoon. You might find that you don't need any salt at all if you use it. If you opt for yellow mustard, which typically has 50 mg of sodium per teaspoon, you will need to add more salt. I would start with ½ teaspoon of sea salt.
You can keep the leftovers in an airtight container in the fridge for up to 4 days. Reheat them in the microwave, covered, at 50% power. You can also freeze the cooled leftovers in freezer bags.