Preheat the oven to 400°F (200°C). Line a large baking sheet with parchment paper.
Trim the bottom of the cauliflower stem just enough to remove the dry end, leaving the core intact to hold the slices together.
Slice the head straight down the middle, then cut 1-inch thick slices from the center halves to create the steaks.
Mix the olive oil, garlic powder, smoked paprika, onion powder, salt, and pepper in a small bowl.
Brush both sides of each cauliflower slice generously with the seasoned oil and place them on the prepared baking sheet.
Roast for 20 minutes, then carefully flip the slices with a wide spatula. Top with Parmesan cheese if using, and roast for another 10 to 15 minutes until crispy and golden brown.