Preheat the oven to 350°F. Spray a 9-inch baking pan with non-stick cooking spray.
Into a large skillet over medium heat, add the butter. When the butter is melted, add the leeks. Cook for 5 minutes, occasionally stirring until softened and lightly caramelized in places. Remove from heat and stir in the asparagus. Set aside until cooled completely.
In a medium bowl, combine the JustEgg, cooled leeks and asparagus, spinach, basil, feta cheese, Parmesan cheese, ground black pepper, and salt. Transfer the mixture to the prepared baking pan.
Bake for 30 minutes, until a toothpick inserted in the middle comes out clean. Cool in the baking pan for 10 minutes then carefully invert the frittata onto a cooling rack. Let iitsit for 15 minutes, then transfer the frittata from the rack to a serving plate. Serve warm or at room temperature.