Using a paring knife, remove center core of cabbage. In a large nonreactive stockpot, bring 2 quarts water to a boil. Add cabbage and cook 2 to 3 minutes or until outer leaves are bright green and tender. Lift cabbage from water, and remove outer leaves. Return cabbage to boiling water, and repeat brief cooking and removal of leaves until all leaves are cooked. Reserve 2 cups cabbage cooking water. Trim thick center vein from bottom of each leaf. Reserve four large outer leaves to line bottom of pan.

In a medium skillet, melt butter over medium heat. Add onion and garlic, and cook until golden and tender, about 8 minutes. In a large bowl, combine onion mixture, rice, beef, pork, salt, pepper, parsley flakes, green pepper, and celery. Stir to combine.

Add about 1/3 cup rice filling to one cabbage leaf. Fold sides of cabbage over filling, and, starting with the stem end, roll the cabbage up. Repeat with remaining leaves and filling.

Line a 5-quart Dutch oven with reserved outside leaves. Transfer stuffed cabbage leaves to Dutch oven.

In a large bowl, combine tomato puree and the reserved 2 cups of cabbage cooking water. Pour some of the tomato sauce over cabbage to almost cover. Sprinkle apple over top of cabbage leaves. Bring to a boil and immediately reduce to a gentle simmer; cover. Cook for 1 hour or until cabbage is very tender, adding additional tomato sauce as needed.

Place sour cream in a small bowl, and ladle in about 3/4 cup of tomato sauce from the cooked stuffed cabbage rolls. Whisk to combine. Add sour cream mixture back to Dutch oven, and stir to combine. Serve with additional sour cream.