Coat a roasting pan or two 17x12x2-inch baking pans with cooking spray. Remove all unpopped kernels from the 12 cups of popped corn. Place the popcorn and nuts in the pan. Keep the popcorn warm in a 300 degree F oven while you're making caramel mixture.
In a 2-quart saucepan, combine the brown sugar, the butter, and the corn syrup. Bring the mixture to boiling over medium heat, stirring constantly (this takes about 12 minutes). Cook and stir the caramel mixture for 5 minutes more over medium heat. Remove the saucepan from the heat; stir in the baking soda (the mixture will foam).
Pour caramel mixture over popcorn in prepared pan; stir gently to coat.
Bake in the center of a 300 degree F oven for 15 minutes. Stir popcorn. Bake for 5 minutes more. Remove the baked caramel corn from oven. Spread onto a large piece of foil to cool. Break apart to serve. Makes about 15 cups.