A towering cheese souffle makes an impressive meatless main dish. For the highest, puffiest souffle, be sure to use a clean bowl and beaters for the egg whites and fold gently when combining the yolk and white mixture to retain as much air as possible in the beaten whites.
1. Heat oven to 400 degrees F. Coat 10-cup souffle dish with cooking spray. Coat with the 3 tablespoons Parmesan.
2. Whisk milk and flour in saucepan. Bring to simmering over medium-high heat, whisking occasionally, until thickened, about 5 minutes. Remove from heat. Whisk in remaining 1/3 cup Parmesan, cheddar, mustard, salt and chives.
3. Whisk small amount of milk mixture into yolks in bowl. Whisk in almost half of milk mixture; whisk yolk mixture into milk mixture in saucepan. Pour into large bowl.
4. In clean bowl, beat whites and cream of tartar on medium until foamy. Increase to high; beat until stiff peaks. Fold half of whites into yolk mixture; fold in remaining whites. Scrape into prepared dish; smooth top. To make top hat on souffle, hold spoon upright and with back side of tip, make circle around mixture, 1 inch from edge of pan.
5. Place souffle in 400 degrees F oven; reduce temperature to 375 degrees F. Bake in 375 degrees F oven 30 to 35 minutes, until lightly golden, puffy and lightly set in center. Serve immediately.