Heat oil in saucepan over medium-low heat. Sauct onion, celery, and bell pepper. Season with salt. Cover and cook for 5 minutes. Add mushrooms and garlic. Cover and cook 5 more minutes.
Stir in stock and rice. Season to taste with pepper. Increase heat and bring to boil. Cover, reduce heat, and cook for 15 minutes or until rice is tender. Remove from heat, fluff rice. sir in parsley and set aside.
Bring 1 inch of water to boil in a large pot. Place cabbage in water, cored side down. Cover and cook for 5 minutes. Remove cabbage and immediately peel off first 4 leaves. Return cabbage to pot and repeat. Cover the leaves with plastic wrap. When the leaves are cool enough to handle, carve off touch ridge from center of leaves.
Preheat oven to 375. Drain sauerkraut in a colander. Rinse and squeeze out liquid. Place sauerkraut in even layers in a 13x9 inch casserole pan. Top with 2 cups of tomato sauce.
Place 3/4 cup of rice in center of each leaf. Fold leaf around rice and place seam side down in casserole dish. Spoon remaining tomato sauce over cabbage.
Can be covered and stored in refrigerator for a day at this point.
Cover with foil and bake until bubbly, about 45 minutes. Check after 30 minutes and if it looks too liquidy remove foil.