Preheat the oven to 425°F (220°C) and line a large baking sheet with parchment paper.
Slice the cauliflower. Remove outer leaves but keep the core fully intact. Stand upright and cut straight down into 0.75 to 1 inch thick slabs. You should get 3 to 4 solid steaks.
Make the spiced oil. Mix olive oil, garlic powder, smoked paprika, cumin, onion powder, turmeric, salt, pepper, and lemon juice in a small bowl.
Season the steaks. Pat the cut surfaces dry with a paper towel, then brush both sides of each steak generously with the spiced oil mixture.
Roast. Lay steaks flat on the prepared baking sheet with space between each one. Bake for 20 minutes, then carefully flip with a wide spatula. Return to the oven for 12 to 15 more minutes until deeply golden and the edges are caramelized.
Rest and serve. Let the steaks rest for 2 to 3 minutes before plating. Finish with lemon juice, fresh parsley, or parmesan if desired.