Heat the oil in a large sautee pan. Add the onions and carrots. Cook the onions for 5-7 minutes, or until the carrots are soft. Add the garlic. Cook for another minute, stirring often. Stir in the cumin and cinnamon.
Add the rice to the pan. Toast the rice, stirring frequently, for a full five minutes; you want it to smell lightly toasted and fragrant. Add the currants and the 2 3/4 cups water. Bring the mixture to a boil. Reduce to a simmer, cover the pan, and cook for 30-35 minutes, until the rice is tender. Stir in the lentils, salt, a generous dash of pepper, the cumin, and the lemon juice.
Preheat your oven to 350F. Oil a 9 x 13 inch baking dish. Spread a thin layer of tomato sauce (about half a cup) over the bottom of the dish.
To prepare the cabbage rolls, bring a large pot of water to a boil. Submerge the whole head of cabbage in the water for 3 minutes. Use tongs to remove the head from the pot, and run it under cold water. Slip the first few outer skins off; they should be very easy to remove in one whole piece. Return the cabbage to the pot for 2 minutes, and then repeat the process. Do this until you have 12 whole, par-boiled cabbage leaves. Pat the leaves gently to dry them, and reserve remaining cabbage to toss into salads or slaws.
Use a paring knife to trim down the thick stems of the cabbage leaves. Add a generous 1/2 cup of the rice lentil mixture to the center of the leaf. Tuck in the bottom and the top, and then fold one side over the filling tightly. Wrap the other side over and around the roll, making sure to keep things snug, and place the whole roll, seam side down, in the baking dish. Repeat with all twelve cabbage leaves. Top the leaves with the remaining tomato sauce.
Bake the rolls for 30-35 minutes, or until the sauce is bubbly and darkened on top of the rolls. Remove from heat and serve.