Coat a 15x10x1-inch baking pan with nonstick cooking spray and set aside.
Combine in a large bowl and stir together the oats, wheat flakes, Grape-Nuts, and nuts.
In a small saucepan, stir together juice, honey, allspice, and cinnamon. Cook and stir just until boiling. Remove from heat. Pour over oat mixture, tossing just until coated.
Spread out mixture evenly in prepared pan. Bake, uncovered, in a 325 degree F oven for 30 to 35 minutes or until oats are lightly browned, stirring twice. Remove from oven. Turn out onto a large piece of foil, cool completely.
Serve with yogurt, milk, or fresh fruit (optional).