1bunch asparagusends trimmed, then cut into 1" pieces (if your asparagus are very thick, also cut in half lengthwise)
Preheat your oven to 375 degrees F.
Add your leek, onion, and asparagus with a tablespoon or two of water to a deep, 12" nonstick, oven safe pan. Sprinkle with salt and pepper and sauté the veggies until softened, about 5 minutes, adding more water as needed to prevent sticking. Remove from heat and set aside.
Meanwhile, combine all remaining ingredients in a large bowl and whisk well to combine. Pour this mixture into your pan and place in the center of your oven. Bake for 40-45 minutes, until edges are brown and the middle is set.
Remove from the oven and let cool for at least 30 minutes before slicing and serving. As the frittata cools to room temperature, it will further set and make slicing easier.
Serve your frittata at room temperature, or chilled.Allow your frittata to rest for at least 30 minutes after baking for it to set. This time will help firm up the frittata, making the texture just right, and also easier to slice.