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 nonstick skillet over medium heat.
Using a 4-tablespoon scoop or measuring cup, transfer four mounds of the tuna mixture to the hot oil.
Gently flatten the mounds with the back of a fork.
Cook the patties until golden brown, about 3 minutes per side. Keep the cooked patties in the warm oven while you fry the second batch.
Serve immediately.
Video
Notes
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 can 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.
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.