Cook the rice according to the package, but leave just undercooked.
Cook the lentils based to the package, also little undercooked.
Add the diced tomatoes, sliced mushrooms and celery into a large-mixing-bowl.
Heat olive-oil in medium frying-pan over low-medium heat. Once hot, add chopped onions and minced garlic. Cook about 5-10minutes or till onions are transparent, stirring periodically.
Then, add the softened garlic & onions into the large-mixing-bowl.
For cabbage, cut 1/4" away from the base of the cabbage. After that, put the entire head of cabbage to boiling water for approximately 3minutes. Peel the softened leaves the cabbage off. Repeat till all of the bigger leaves readily come off.
The lentils and rice should be completed, so go right ahead and add them into the large-mixing-bowl.
Preheat oven to 350°F. Then add approximately 1 cup of tomato sauce into the bottom of the casserole-dish. Put aside.
Lay the cabbage leaf flat on a cutting board. Add about two large-spoonfuls into the middle of the cabbage. Next, fold both sides over the filling, then roll the cabbage. Place seam side down in the casserole-dish. Repeat with the remaining leaves.
Subsequently, spread the remaining tomato sauce on the top of the cabbage rolls. Then cover and cook about 70minutes.
Serve and enjoy.