Melt 1/2 cup of butter in a saucepan over medium heat. Add the onion; cook and stir until deep brown, about 15 minutes. Stir in the maple syrup and trail mix. Heat to 300 to 310 degrees F (149 to 154 degrees C), or until a small amount of syrup drizzled from a spoon forms hard, brittle threads. Pour onto parchment paper, and allow to cool.