Trim cauliflower stalk and place head stalk side down on a cutting board. Slice cauliflower lengthwise into 1/2" slices. Arrange cauliflower slices and any stray pieces in a heavy pan. Heat butter over medium heat and sear cauliflower steaks on both sides.

Remove cauliflower steaks from pan and set aside. Melt additional butter over medium heat. Add pumpkin seeds, bring to a simmer, and cook, swirling pan occasionally, until pumpkin seeds are toasted and butter is melted. Remove from heat and re-add steaks to pan. Add chopped parsley and season with salt and pepper. Drizzle steaks with melted butter from pan.