Preheat the oven to 350°F. Make sure to move the rack to the upper third rack.
Melt butter in a large cast iron pan on medium heat. Add onion and cook for 5 minutes, stirring frequently.
Turn the heat to medium-high, add cabbage in two batches, saute for 5 minutes to get some color, you want the cabbage to get a nice char. Then add the other half of the cabbage and cook for another 5 minutes.
Season with salt (you want it to be seasoned well, so adjust to your liking), smoked paprika and garlic powder, taste if you need more seasoning. Add heavy cream. Mix together. Bake for 10 minutes.
While the cabbage is cooking, prepare the panko mixture. Add melted butter, panko, garlic powder, parmesan cheese and mix well to combine.
Remove from oven, and sprinkle with panko mixture. Place back in the oven for an additional 10-15 minutes. Remove from the oven and sprinkle with chopped parsley. Serve warm.