Bring a large pot of salted water to a boil.
Carefully place the cabbage in the water and gently turn with a mesh strainer or wooden spoon.
Every few minutes, with the strainer or spoon, loosen a few leaves and carefully remove.
Set aside to cool.
Do this until the cabbage leaves are softened and removed.
While the cabbage is being cooked you can mix together the filling.
In a large mixing bowl, combine the beef, pork, garlic powder,pepper, crushed red pepper,salt, sugar, 1/2 of the onion and rice.
Mix until combined well.
In a small mixing bowl, whisk together the ingredients for the sauce.
Slice remaining cabbage and onion and layer the bottom of your slow cooker.
Cover with 1/3 of the tomato sauce.
When the cabbage is cool enough to touch, with a knife, turn the cabbage over and remove the stem being careful not to pierce through the cabbage leaf.
Fill the cabbage with about 1/2 cup of filling and roll gently, not too firmly, while tucking in the sides.
Line them across the sauce in the slow cooker.
Cover with another 1/3 of the sauce.
Place your final layer of cabbage rolls in the slow cooker and cover with remaining sauce.
Place the lid on the slow cooker and cook on Low for 8 to 9 hours or on High for 5 hours.
When the it is done cooking, remove the lid and let cool for 15 minutes or so.