Butter a 1 1/2-quart souffle dish and dust it with 1/4 cup of the Parmesan. In a large heavy saucepan cook the onion and the garlic with salt and pepper to taste in the butter over moderately low heat, stirring, until the onion is softened, stir in the flour, and cook the roux, stirring, for 3 minutes. Add the milk in a stream, whisking, and simmer the mixture, whisking, until it is thickened. Remove the pan from the heat, whisk in the Gruyere, whisking until the cheese is melted, and whisk in the sweet potatoes and the egg yolks, 1 at a time. In a bowl with an electric mixer beat the egg whites with a pinch of salt until they just hold stiff peaks, whisk one fourth of them into the sweet potato mixture to lighten it, and fold in the remaining whites gently but thoroughly. Pour the mixture into the prepared souffle dish, sprinkle the remaining 1/4 cup Parmesan over it, and bake the souffle in the middle of a preheated 375F. oven for 45 to 50 minutes, or until it is puffed and golden. Serve the souffle immedi ately. Serves 4 to 6. Gourmet October 1992