Start by preheating your oven to 400°F, or set your air fryer to the same temperature if you're using one instead. While it heats up, line a large rimmed baking sheet with parchment paper. This keeps the cheese from sticking once it melts and makes cleanup effortless once you're done cooking.
In a small bowl, whisk together the olive oil, grated parmesan, onion powder, garlic powder, paprika, salt, and pepper. This blend is what gives the cauliflower its deep, savory flavor once roasted. Stir it well so the spices don't clump together in one spot.
Remove the outer leaves from the base of the cauliflower, but leave the core intact since it holds the steaks together. Slice the head in half through the stem, then slice each half in half again to create four thick steaks. Keeping the core attached is the key step that stops them from falling apart on the pan.
Transfer the cauliflower steaks onto your prepared baking sheet. Using a pastry brush, coat every side of each steak generously with the seasoned oil mixture, making sure to get the tops, bottoms, and edges. Don't rush this part, since the oil is what carries most of the flavor into the vegetable.
Place the pan in the oven and roast for about 20 minutes, or air fry for about 15 minutes, flipping halfway through if using the air fryer. You're looking for the stalk to become fork-tender and the edges to turn a deep golden brown. This is what gives the steaks their satisfying texture.
Once the cauliflower is tender, pull the pan out and sprinkle the shredded mozzarella evenly over each steak. Slide it back into the oven for just a few more minutes, until the cheese is fully melted and slightly bubbly. Garnish with fresh parsley right before serving for a pop of color.