If making from scratch, prepare tomato sauce by placing all ingredients in a heavy-bottom pan and simmering over medium-low heat ~45 minutes. Turn off heat, taste for salt, then set aside until ready to use.
Cook rice according to package directions but be sure to slightly under-cook it, since it will finish cooking in the oven. (Note: 2 cups of my jasmine rice cooked with 3 cups of water yielded about 5 cups of rice).
While rice cooks, add 2 Tb. butter and the chopped onion to a medium saute pan and cook over low heat until onions are translucent, 3-4 mins. Turn off heat then add ~3 cups of the partially cooked rice and mix with 1/2 tsp. salt and a few grinds of black pepper. Set aside until ready to use.
Heat oven to 350 degrees.
Fill a large pot with water, bring to boil then reduce to simmer.
Fill a large bowl with cold water; set aside.
Remove as many outer leaves of the cabbage as you easily can. Carefully add the head of cabbage to the water and let it sit for ~1 minute. Remove with tongs then drop into the bowl of cold water. When cool enough to handle, pull off more leaves, trying to get 12 whole leaves. If you're having difficulty, pop it back in the water and repeat. Transfer leaves to a dry kitchen towel.
Line the bottom of an 8X8 baking dish with about 1/2 cup of tomato sauce.
To assemble: cut each leaf in half and remove any tough/thick parts of the stem. Place a heaping tablespoon of the rice/onion mixture near the bottom of the leaf. Roll up while pinching in the sides to form a little roll/bundle. Place each roll in the baking dish so they fit snuggly together. After the first layer, spoon on some tomato sauce. Repeat for the second layer. Top with more sauce, then cover with foil and bake about 1 hour, or until the cabbage leaves are tender when tested with a knife. Allow to cool slightly, then serve.