Preheat oven to 400°F (200°C). Line a baking sheet with parchment paper or a silicone baking mat. Thaw the frozen spinach and squeeze out as much liquid as possible using a clean kitchen towel or paper towels.

In a medium bowl, combine the squeezed spinach, crumbled feta, grated Parmesan, beaten egg, chopped green onion, minced garlic, lemon zest, dried oregano and black pepper. Stir until well combined, tasting and adjusting seasoning carefully.

Use a tablespoon or small cookie scoop to place heaped mounds onto the prepared baking sheet. Flatten each mound slightly with the back of the spoon into a 2–2.5 inch disk.

Lightly brush each disk with olive oil and sprinkle sesame seeds on top if desired to promote browning and add texture.

Bake for 10–12 minutes or until edges are golden and centers are set. For an extra-crisp finish, broil for 1–2 minutes, watching closely to prevent burning.

Remove from the oven and cool for 2–3 minutes on the baking sheet before transferring to a wire rack. Serve warm with tzatziki, lemon wedges or as part of a mezze board.