Preheat oven to 400 degree F. Peel and core pears; cut into large chunks. In a very large bowl stir together the 1/4 cup granulated sugar and the cornstarch. Add pears, blueberries, and lemon juice, stirring gently to combine. Transfer to a 3-quart rectangular baking dish.
In a food processor bowl or large mixing bowl combine flour, the 1/2 cup granulated sugar, and brown sugar. Pulse with several on-off turns or cut in butter until mixture is crumbly. Sprinkle atop fruit. Bake for 30 to 35 minutes or until top is lightly browned and syrup is bubbly. Remove from oven. Drizzle with cognac, if desired. Let stand about 40 minutes before serving. In a small chilled bowl beat whipping cream and powdered sugar together with chilled beaters on medium speed until soft peaks form. Serve with crisp. Makes 10 servings.