Cheesy Onion Cornbread

1 large Spanish onion
2 tablespoons butter or margarine
1 1/2 cup cornbread mix
1 (8 ounce) can cream-style corn
1/3 cup milk
1 egg, beaten
1 cup (4 ounces) shredded sharp Cheddar cheese
3 ounces finely diced pepperoni (optional)
1 cup sour cream
1/4 teaspoon salt

Slice onion into thin rings.

Melt butter in small skillet. Add onion. Saut