Step 1Place a rack in the upper third of the oven and heat the oven to 325 degrees.
Step 2In a large bowl, mix together the leeks, spinach, lettuce, green onions, parsley, cilantro and dill. Set aside.
Step 3In a small bowl, stir together the flour and water to make a paste. Set aside.
Step 4In a bowl, whisk the eggs until smooth. Add the yogurt, the flour and water paste, salt and pepper, stirring to combine.
Step 5Heat a 10-inch cast-iron or other heavy oven-proof skillet over medium heat. Add the oil, coating the bottom of the pan. Remove from heat.
Step 6Pour the egg mixture over the greens and stir, then pour the mixture into the hot pan. Sprinkle over the walnuts. Bake until the frittata is cooked through and set, about 45 minutes. Set aside for an hour or more to cool at room temperature before cutting into wedges and serving.