Pulse first 3 ingredients in a food processor 3 to 4 times or until mixture is combined.
Add 9 tablespoons butter; pulse 5 to 6 times or until crumbly. Add 1 egg, and process until dough forms a ball and leaves sides of bowl. Shape dough into a disc; wrap in plastic wrap, and chill 30 minutes.
Press dough into bottom and up sides of a 10-inch tart pan. Prick bottom with a fork. Chill 20 minutes.
Line dough with greased aluminum foil, and fill with pie weights or dried beans.
Bake at 350° for 15 minutes. Remove weights and foil; bake 15 more minutes or until lightly browned.
Whisk together remaining 6 eggs, lemon rind, and lemon juice in a non-aluminum medium saucepan over low heat.
Add sugar and 6 tablespoons butter, and cook, whisking constantly, 8 minutes or until lemon mixture is thickened and bubbly. Let cool 15 to 20 minutes. Pour filling into prepared crust; cover and chill 4 hours or until set. Serve with whipped cream, if desired. Garnish, if desired.
