Cut out the center core of the cabbage, leaving the cabbage whole.
Fill a large (6-quart) pot of water four inches from the top. Bring to a boil over high heat.
Use a tongs to submerge the cabbage in the boiling water and blanch it for about three minutes, allowing the outer leaves to soften. Remove the cabbage from the water. Use the tongs and your fingers to remove the outer leaves of the cabbage and put them aside on a plate to cool. Return the head of cabbage to the water, repeating this process until you have peeled off all of the softened, pliable cabbage leaves.
Heat the oil in a medium (10-inch) skillet over medium heat. Add the onion and salt, then sauté for 10 minutes, until the onions are translucent and softened but not quite beginning to brown. Transfer the onions to a medium mixing bowl and allow to cool to room temperature. When the onions have cooled, stir in the pepper, ground beef, egg, and cooked rice until thoroughly combined.
Ladle 1/2-cup of the tomato sauce into a 9-inch square baking dish and spread it out evenly.
When rolling the cabbage leaves, cut the larger leaves in half down the center, removing the thick center vein. Smaller leaves can be left whole. Scoop three tablespoons of filling onto the bottom third of each leaf, then tuck in the sides and roll it up like a burrito or egg roll. Place each roll seam side down in the baking dish as you go, making two layers of rolls to fill up the dish. When you have rolled all of the leaves, spoon the remaining cup of crushed tomatoes over the top. Cover the dish tightly with aluminum foil.
Bake for one hour, then allow to cool for 15 minutes before serving.