Sweet Potato Soufflés

Ingredients

• 2 lbs. sweet potatoes
• 1 tablespoon plus 2 tablespoons unsalted butter
• 1/4 cup plus 1/4 cup grated Parmesan cheese
• 1 medium white onion, minced
• 3 tablespoons all-purpose flour
• 1 1/2 cups milk
• 1/4 teaspoon freshly grated nutmeg
• 1/4 teaspoon allspice powder
• 1/4 teaspoon ginger powder
• 6 large eggs, separated
• 1 1/2 cups coarsely grated Gruyere, 4 ounces