Grate the courgette on the large holes of a grater onto a clean kitchen towel. Sprinkle with the salt and let it rest while you gather and prep the remaining ingredients.
Wrap the courgette in the towel and wring as much liquid out of it as possible, discarding the liquid. In a medium bowl, combine the courgette, feta, dill, mint, garlic, spring onion and all but 1 teaspoon of the lemon zest. Stir in the flour and egg and mix until well combined.
Add the oil to a large shallow pan, about 1/4 inch, or enough so that when all the fritters are in the pan, the oil comes halfway up their sides. Place the pan over medium-high heat. Form fritters by hand or using a 1/4-cup measure, and fry them in the hot oil in batches. Cook until the fritters are golden brown on each side, 4 to 6 minutes. Drain on paper towels.
Transfer the fritters to a serving platter and garnish each with a dollop of Greek yoghurt and a small sprig of dill. Sprinkle the reserved lemon zest and some coarse sea salt over the serving platter. Serve hot or at room temperature.