In a large mixing bowl, combine the drained tuna, breadcrumbs, chopped green onions, and minced garlic. Mix well.
In another bowl, whisk together the egg, mayonnaise, Dijon mustard, lemon juice, and lemon zest until smooth.
Pour the egg mixture into the tuna mixture and stir until fully combined. Season with salt and pepper to taste.
Form the mixture into patties, about 2-3 inches in diameter and 1/2 inch thick.
Heat a skillet over medium heat and add a drizzle of olive oil to coat the bottom.
Once the oil is hot, carefully place the tuna cakes in the skillet, cooking in batches if necessary. Fry for about 4-5 minutes on each side, until golden brown and crispy.
Remove the cakes from the skillet and place them on a paper towel-lined plate to absorb any excess oil.
Serve warm, garnished with fresh parsley if desired.