Sweet Potato Souffle

My son, Christopher, will tell you he despises sweet potatoes. However when this dish made its debut on my Thanksgiving table he became its biggest fan. It has since replaced the sweet potatoes with mini marshmallows that my mom always made.
This dish begins with a puree of sweet potato. You can make the puree way in advance and freeze until you are ready to assemble the dish. I make the first part of the dish on Thanksgiving morning, saving the addition of the egg whites until just before placing it in the oven.

Puree
3 large sweet potatoes
5 tablespoons maple syrup
2 tablespoons melted butter
Souffle
2 tablespoons butter
1 small onion diced
2 cloves of garlic minced
salt and pepper
1 teaspoon dried thyme
2 tablespoons flour
1 cup milk
1 1/2 cup shredded gruyere cheese
2 cups sweet potato puree
6 large eggs, separated

To Make Puree Preheat the oven to 400. Pierce the potatoes with a knife and place on a lined baking sheet. Bake for 1 hour or until a knife inserted into a potato goes in easily. Cool until you can handle them. Peel away the skin and place the potatoes into a food processor. Add the maple syrup and butter. Puree until smooth. Freeze until ready to use.

To make the souffle
Preheat the oven to 375. Spray a 2 quart casserole with cooking spray. In a medium saucespan, melt the butter over medium heat. Add the onion, garlic and thyme. Season with salt and pepper. Cook stirring until the onion has softened. Add the flour and cook for 2 minutes, stirring continuously. Slowly whisk in the milk until well combined. Switch back to a spoon and stir until the mixture thickens and comes to a simmer. Remove from the heat and add the cheese and 1 teaspoon of salt. Whisk until the cheese has melted and the sauce is smooth. Whisk in the sweet potato puree. Add the egg yolks one at a time whisking after each addition.(If you are making ahead of time, you can stop at this point and set aside until 45 minutes before you are wanting to serve the dish.)In a clean mixing bowl, beat the egg whites with a pinch of salt until stiff peaks form. Using a rubber spatula, gently fold the egg whites into the sweet potato mixture until just combined. Pour into the casserole and sprinkle with the remaining cheese. Bake until puffed and golden for 40-45 minutes. Serve immediately