SAVORY WINTER SQUASH SOUFFLE

This was adapted a little from one by Amy Topel, a chef who has a nice website with seasonal recipes. I used sweet potatoes, and pureed them in the food processor to remove the stringiness. For more details on making souffle, check out SPINACH & CHEESE SOUFFLE.
1/2 cup freshly grated Parmesan
1 cup finely chopped onion
2-1/2 Tbsp unsalted butter
2 Tbsp all-purpose flour
1 cup milk, heated to almost boiling
1/2 cup grated Fontina cheese
1-1/2 pounds winter squash or sweet potatoes, cooked and mashed
4 large eggs, separated
1/4 teaspoon cream of tartar

Butter a 1-1/2-quart soufflé dish, dust with 1/4 cup of the Parmesan and set aside. Preheat oven to 375 degrees. In a saucepan over low heat, sauté onion in butter until translucent. Add flour and stir constantly for 3 minutes. Slowly add milk, whisking constantly, until the mixture has thickened. Remove from heat, whisk in Fontina, mashed squash and egg yolks until completely smooth. Beat the egg whites with a pinch of salt. As they begin to froth, add the cream of tartar and then continue to beat until they form stiff peaks. Gently fold into the squash mixture. Pour into soufflé dish, sprinkle with remaining Parmesan and bake for 30 -40 minutes, or until puffed and golden. Serve immediately.