1Heat oven to 425°F. Spray 15x10x1-inch pan with cooking spray. Place squash in single layer in pan. Bake 40 minutes, stirring after 20 minutes, or until tender. Cool 5 minutes. Reduce oven temperature to 350°F.
2Spray 8 (4-oz) custard cups or ramekins with cooking spray. Place cups on cookie sheet. In food processor, place squash, cheese, flour, butter, milk, sage, salt and egg yolks. Cover; process, using quick on-and-off motions, until well blended. Spoon squash mixture into large bowl.
3In medium bowl, beat egg whites with electric mixer on high speed until stiff peaks form. Fold into squash mixture. Divide evenly among custard cups.
4Bake 35 to 40 minutes or until slightly puffed and golden brown. Sprinkle with walnuts. Serve warm.