You want to start with super smooth sweet potato puree, so I suggest using a food processor for best results.
2 large sweet potatoes, roasted, peeled & flesh scooped out and pureed
1 leek, sliced
4 tablespoons of butter, softened
1 cup ricotta (if using store bought, drain any excess water)
4 egg yolks
salt and pepper
4 egg whites
Preheat oven to 350ºF. Grease the bottom and sides of a 9-inch ceramic or glass pie plate with butter; set aside.
Heat 2 tablespoons of butter in a skillet over medium heat. Add leeks and saute until tender and fragrant; set aside. Meanwhile, using the whisk attachment on your stand mixer, beat the egg whites until stiff peaks form; set aside.
In a separate medium-sized bowl, mix the sweet potato puree, butter, ricotta, egg yolks and salt pepper together. Stir in leeks. gently fold in egg whites. Spoon into prepared pie dish and bake for 1 hour, until puffed and golden. Serve immediately.