Preheat oven to 350 degree F. Grease a 13x9x2-inch baking pan; set aside. In a 15x10x1-inch baking pan combine 1/2 cup of the walnuts, 1/2 cup pecans, 1/2 cup almonds and all of the rolled oats. Bake about 12 minutes or until toasted, stirring two to three times. Set aside on wire rack.
In a large mixing bowl stir together the flour, granulated sugar, baking powder, salt, and baking soda. Stir in the zucchini, oil, eggs, and vanilla until combined. Stir in toasted nut mixture. Spread into prepared baking pan.
Bake for 35 to 40 minutes or until a wooden toothpick inserted near the center comes out clean. Cool in pan for 1 hour on a wire rack.
In a small saucepan combine butter and half-and-half. Cook and stir until butter melts. Add brown sugar; stir until sugar dissolves. Remove from heat. Stir in remaining nuts.
Spread nut mixture over cake in pan. Broil 4-inches from heat for 1-1/2 to 2 minutes or until topping is bubbly and golden. Cool in pan on wire rack before serving. Makes 20 servings.