Preheat broiler and set a rack 6 inches from the flame. Crack eggs into a large bowl; add salt, pepper and grated Parmesan. Lightly scramble and set aside.

In a large oven-safe pan, heat oil and butter over medium-high heat. Add the leek; saute for 1 minute, stirring occasionally. Add the asparagus; saute for 3 minutes, or until almost fully cooked. Add the spinach and season with salt and pepper to taste. When the spinach is almost fully wilted, spread the mixture across the bottom of the pan and pour the eggs over the top. Turn the pan so that the eggs fully cover the vegetables.

When the edges of the eggs start to separate from the sides of the pan, turn off the heat. Place the pan under the broiler until the frittata is lightly golden and the eggs are fully set, about 5 minutes. Immediately turn the frittata onto a large serving plate and garnish with extra Parmesan, if desired. Cut into slices and serve.