Preheat oven to 400°F (200°C) and line a baking sheet with parchment paper.
Remove the green leaves from the cauliflowers and wash them very well. Using a clean towel, pat dry very well. Trim the stem, but leave the cores intact to hold the steaks together.
Place the cauliflower upright over a flat surface, then slice it into ¾-inch-thick slices. You should get 2 to 3 steaks from each head, with some leftover florets.
In a small bowl stir together the olive oil with all the seasonings. Use a kitchen brush or fork to coat both sides of the cauliflower steaks with the seasoned oil mixture.
Line the seasoned steaks on the prepared baking sheet. Roast in hot oven for 20 to 25 minutes in total, flipping them after 10 minutes. You know they are ready when tender if poked with a fork.
Serve warm as a side dish or as a main, garnished with chopped parsley and cilantro if desired.