Take out the core of the cabbage head, then, submerge the cabbage in the boiling water for a few minutes until some of the leaves start to peel away and the leaves begin to soften, in a way that each leaf must be pliable enough for rolling later.
Remove from water and cut off the thick part of the leaves to flatten it.
Prepare the filling:
Combine the "Cabbage Rolls" ingredients, except the cabbage, in a large bowl.
Make the sauce:
Spray the slow cooker with cooking spray.
Layer 3 cabbage leaves on the bottom of the slow cooker.
Add the sauce ingredients to a bowl, stir to combine and transfer HALF to the slow cooker.
Add about 2-3 tablespoons of filling inside a cabbage leaf and roll. Make sure to seal both ends. Repeat with the remaining leaves. Set aside.
Layer the cabbage rolls in the slow cooker, and pour the remaining sauce on top.