Heat the oven to 375 F. Lightly coat 6 (8-ounce) ramekins with cooking spray and sprinkle with panko breadcrumbs. Place the ramekins on a baking sheet.
Heat a medium saute pan over medium heat. Add the sweet potatoes, butter, thyme, salt and pepper, and nutmeg.
In a small bowl, combine milk and cornstarch to form a slurry. Add the slurry to the pan and bring to a light boil while whisking frequently. Reduce heat and stir in the cheese. Once the cheese is melted, remove from heat and let the mixture cool.
In a medium bowl, whip the egg whites with an electric mixer until firm peaks form. Carefully fold the sweet potato mixture into the egg whites. Place equal amounts of the mixture into the ramekins and bake for approximately 20 minutes or until the center is firm and slightly golden brown.