Preheat oven to 400 degrees. Butter a 11x7-inch baking pan.
In a large mixing bowl, stir together corn muffin mix, egg and milk until combined. Add cheese, onion and kielbasa and stir to combine.
Pour batter into prepared pan and bake in preheated oven for 30 minutes until a toothpick inserted in the center comes out clean. Remove from oven and allow to cool for 10 minutes before serving.