Process first 4 ingredients in food processor until dough forms ball. Roll into a 12-inch circle. Place dough in 10-inch tart pan; trim edges, leaving 1-inch overhang. Fold overhang inside pan, and press against sides, forming a thick edge. Line pastry with aluminum foil. Fill with pie weights or dried beans.
Bake at 375° for 18 minutes. Remove from oven, and place on a wire rack to cool while preparing filling. Reduce oven temperature to 350°.
Combine sugar, water, and lemon juice in a medium saucepan. Bring to a boil over medium-high heat. Cook, stirring occasionally, 15 minutes or until mixture turns a dark amber color. Wash down sides of pan once with a small brush dipped in hot water. Remove from heat, and carefully stir in cream and butter until smooth.
Place nuts in bottom of tart shell. Place tart on a baking sheet. Pour caramel mixture over nuts. Bake at 350° for 15 minutes. Carefully remove tart from oven, and place on a wire rack. Cool completely. Serve with whipped cream, if desired.