In a large bowl, combine the flour, sugar and salt. Cut in butter until mixture resembles coarse crumbs. Press half of the crumb mixture into a greased 15-in. x 10-in. x 1-in. baking pan. Bake at 375° for 10-12 minutes or until lightly browned. Set remaining crumb mixture aside.
Meanwhile, in a large saucepan, bring zucchini and lemon juice to a boil. Reduce heat; cover and simmer for 5-6 minutes or until tender. Drain. Stir in the sugar, cinnamon, nutmeg and 1/2 cup reserved crumb mixture. Cook and stir for 2-3 minutes. Stir in walnuts and raisins.
Spread filling evenly over crust. Sprinkle with remaining crumb mixture. Bake for 25-30 minutes or until golden brown. Cool on a wire rack. Cut into squares.