Prepare your slow cooker by lightly greasing the bottom with cooking spray.
Layer half of the cabbage wedges in the bottom of the slow cooker, cut side down.
Scatter half of the sliced onions over the cabbage layer.
Dot with half of the butter pieces, distributing evenly.
Season with half of the salt, pepper, and garlic powder.
Repeat the layering process with remaining cabbage, onions, butter, and seasonings.
Pour the broth around the edges of the slow cooker (not directly over the top).
Cover and cook on LOW for 5 hours or HIGH for 2.5-3 hours, until cabbage is fork-tender.
Toss gently before serving, allowing the butter and juices to coat all the pieces.