01 - Preheat oven to 425°F. Line a rimmed baking sheet with parchment paper and set aside.
02 - Remove outer leaves from each cauliflower head and trim the woody stem end so it sits flat. Standing the head upright, slice vertically into 3/4-inch thick steaks, yielding 2 to 3 steaks per head. Reserve any loose florets for another use.
03 - Place the cauliflower steaks in a single layer on the prepared baking sheet. Brush both sides generously with the melted butter and olive oil using a pastry brush.
04 - Sprinkle both sides evenly with garlic powder, smoked paprika, kosher salt, and freshly ground black pepper.
05 - Roast in the center of the oven for 20 minutes, carefully flipping the steaks once at the halfway mark, until the edges are golden and the cores are fork-tender.
06 - While the cauliflower roasts, combine the shredded mozzarella, cheddar, and grated Parmesan in a mixing bowl. Toss to distribute evenly.
07 - Remove the baking sheet from the oven. Mound a generous, even layer of the cheese blend over each steak. Return to the oven and bake for 8 to 10 additional minutes, until the cheese is fully melted, bubbling, and golden in spots.
08 - Transfer the steaks to a serving platter. Scatter chopped parsley over the top, if desired, and serve immediately while the cheese is still bubbling.