Grate the zucchinis using a box grater. Place the grated zucchini in a clean kitchen towel or cheesecloth and squeeze out as much moisture as possible.
In a large mixing bowl, combine the grated zucchini, chopped spinach, crumbled feta, Parmesan cheese, green onions, dill, eggs, flour, baking powder, salt, and pepper. Mix until well combined.
Heat 1–2 tablespoons of olive oil in a large skillet over medium heat. Scoop about 2 tablespoons of the mixture per fritter and drop it into the skillet. Flatten slightly with a spatula to form patties. Cook the fritters for 2–3 minutes per side, or until golden brown and crispy. Transfer the cooked fritters to a paper towel-lined plate to drain excess oil.
Grate the cucumber and squeeze out excess moisture. In a small mixing bowl, combine the Greek yogurt, grated cucumber, minced garlic, olive oil, lemon juice, dill, salt, and pepper. Stir until smooth and well combined. Chill in the refrigerator until ready to serve.
Arrange the warm fritters on a platter and serve with a generous dollop of garlic tzatziki on the side.
