Preheat the slow cooker, if necessary, or according to the manufacturer's directions.
First make the tomato sauce. Heat the oil in a heavy pan. Add the onions and cook over medium heat, stirring occasionally, for 5 minutes until softened. Stir in the garlic and cook for 1 minute, then add the tomatoes, tomato paste, sugar, and bay leaf. Season with salt and pepper and bring to a boil. Lower the heat and simmer gently for 20 minutes until thickened.
Meanwhile, mix the nuts, onions, garlic, celery, cheese, and thyme in a bowl. Lightly beat the eggs with the yeast extract in a pitcher, then stir into the nut mixture.
Cut out the thick stalk from the cabbage leaves. Blanch the leaves in a large pan of boiling water for 5 minutes, then drain, and refresh under cold water. Pat dry with paper towels. Place a little of the nut mixture on the stalk end of each cabbage leaf. Fold the sides over, then roll up to make a neat package.
Arrange the packages in the slow cooker, seam side down. Pour the sauce over the cabbage packages. Cover and cook on low for 3-4 hours. Serve the cabbage packages hot or cold.