We like baking this dish individual ramekins. There's something very satisfying and personal about scooping bites from one's own little bowl. We've also made this sformato in an 8x8 baking dish and 2-liter casserole dish, and extended the cooking time by 10-15 minutes.
Sweet Potato Sformato
1 1/2 cup grated asiago cheese, loosely packed
1.5 pounds (2 average-sized) sweet potatoes
3 cups whole or 2% milk
4 tablespoons unsalted butter, plus extra for greasing the ramekins
1/4 cup sweet rice flour (or regular all-purpose flour)
1 teaspoon salt
1 tablespoon minced thyme
1/4 teaspoon freshly grated nutmeg
1/2 teaspoon black pepper
2 large eggs
1 large yolk
Heat the oven to 400°F. Rub the insides of six 8-ounce ramekins with butter and sprinkle with 1/4 cup of the asiago cheese. Cheese should stick to the bottom and sides of the ramekin in a single layer.
Prick the sweet potatoes all over with a fork and place them on a microwave-safe plate lined with a paper towel. Microwave on high in 3-minute intervals, turning the potatoes between each interval and testing them for doneness. It should take 9-12 minutes total for the sweet potatoes to completely soften. Allow the potatoes to cool enough to handle. (Alternatively, you can wrap the potatoes in foil and roast them in a 400°F oven until soft.)
Meanwhile begin making the bechamel. Warm the milk in a small saucepan until it just barely starts to steam (or microwave to warm). Remove from heat. In another pan, melt the butter over medium heat. Add the flour and stir until a thick paste is formed, 3-5 minutes. Add the milk a little at a time while whisking the butter-flour mixture. Wait to add more milk until the mixture becomes smooth and creamy again. Once all the milk is added, simmer and stir the bechamel until it starts to thicken. Remove from heat and set aside to cool to room temperature.
Peel the sweet potatoes and mash the insides in a large mixing bowl. Add one cup of the cheese (reserving the last 1/4 cup for topping), the thyme, the nutmeg, and the black pepper. Stir in the cooled bechamel sauce a little at a time to achieve a smooth pudding-like consistency. Before adding the eggs, taste the mixture and add more salt or pepper if desired. Whisk the eggs and yolks together and stir them into the sweet potato mixture.
Place all the ramekins on a baking sheet (a silpat can help keep them from slipping around). Fill the ramekins to within 1/2 inch of the top. Bake for 35-40 minutes, until the tops are puffed and beginning to brown in spots. Five minutes before the end of cooking, sprinkle each sformato with a little of the reserved cheese.
Allow the sformatos to cool for about 10 minutes before serving. Leftovers can be kept refrigerated for up to a week and do well reheated for a few minutes in the microwave or a toaster oven.
• Vegetable Substitutes - Try substituting 2 cups of vegetable puree for the sweet potatoes. Squash, cauliflower, and even mushrooms work very well.
• Alternate Spices - We make a fantastic southwestern version of this dish with a few teaspoons of chili powder. Sage and rosemary are also very good in place of the thyme.
• Make it a Meal - To make the sformato more filling, you can add a spoonful of cooked sausage, crumbled bacon, or sautéed mushrooms to each ramekin before filling them.