Place the rack in the middle of the oven and preheat it to 210 °C / 410 °F.
sauté the vegetables
Cut the vegetables into small pieces. Place a 24 cm or 9-inch castiron pan over medium heat. Add oil and butter. Add the leek, spring onion, and peas when the butter melts. Sauté for 8 minutes. Add the baby spinach and cook for 2 minutes.
beat the eggs
Combine eggs, chopped herbs, 150 (6 ounces) of feta cheese, and Greek yogurt in a large bowl. Season with pepper.
make the frittata
Season the vegetables with salt. Pour in the egg mixture. Reduce the heat to low heat and cook for 2 - 3 minutes for the eggs to cook, stirring consistently. Crumble the rest of the feta cheese on top. Transfer to the oven. Cook for 10 - 12 minutes at 210 °C / 410 °F. Cut the frittata into slices and serve.