1. Cook the rice according to the package, but leave just undercooked.
2. Cook the lentils according to the package, also a little undercooked.
3. Add the diced tomatoes, sliced celery and mushrooms to a large mixing bowl.
4. Next, heat olive oil in medium frying pan over low-medium heat. When hot, add chopped onions and minced garlic. Cook for 5-10 minutes until onions are transparent, stirring occasionally.
5. Then, add the softened onions and garlic to the large mixing bowl
6. For the cabbage, cut about 1/4" off the bottom of the cabbage. Then, place the whole head of cabbage into boiling water for about 3 minutes. Peel the softened leaves off the cabbage. Repeat until all the larger leaves easily come off.
7. The rice and lentils should be finished, so go ahead and add them to the large mixing bowl as well.
8. Preheat oven to 350° F.
9. Add about 1 cup of tomato sauce to the bottom of the casserole dish. Set aside.
10. Lay the cabbage leaf flat on a cutting board. Add about two large spoonfuls to the center of the cabbage. Next, fold in both sides over the filling, then roll the cabbage up. Place seam side down in the casserole dish. Repeat with the remaining leaves.
11. If there is leftover filling stuff it around the cabbage rolls
12. Then, spread the remaining tomato sauce over top of the cabbage rolls.
13.Cover, and let cook for 70 minutes.