01 - Preheat the oven to 425°F. Line a rimmed baking sheet with parchment paper and set aside.
02 - Remove the outer leaves from each cauliflower head and trim the stem flush with the bottom. 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 olive oil and melted butter.
04 - Sprinkle both sides evenly with garlic powder, smoked paprika, salt, and black pepper.
05 - Roast in the oven for 20 minutes, flipping the steaks once halfway through, until the edges are golden and the centers are fork-tender.
06 - While the cauliflower roasts, toss together the shredded mozzarella, shredded cheddar, and grated Parmesan in a small mixing bowl until evenly blended.
07 - Remove the baking sheet from the oven. Pile a generous, even layer of the cheese blend over each cauliflower steak.
08 - Return the sheet to the oven and bake for 8 to 10 additional minutes, until the cheese is fully melted, bubbling, and golden on top.
09 - Transfer the steaks to a serving platter, garnish with chopped fresh parsley if desired, and serve immediately.