Place 1/2 stick butter in small skillet, melt butter and add diced onion. Cook until onion turns translucent. Then put in bowl and combine cornbread mix, milk, sour cream, cheddar cheese, salt, and egg. Mix well. Then pour into well greased muffin pan or 8x8 pan. Bake at 450 degrees for 10-20 minutes depending on your oven.