Prep: Preheat the oven to 350 F and set a baking sheet aside. You will need to use a large cast iron pan or oven-safe pan that is at least 12” wide; see the recipe notes if your pan is smaller or not oven safe.
Cut the Cauliflower: Wash each head of cauliflower well. Use a paring knife to carefully cut off the green leaves at the bottom of each head, keeping the central stem intact. Turn the cauliflower stem-side up, then carefully cut on either side of the stem. Cut the stem down the center to create two cauliflower “steaks.” Repeat with the other head of cauliflower; save the florets you chopped off for another recipe!
Make the Spice Rub: Add 1 tablespoon of oil to a small bowl along with the garlic powder, cumin, paprika, and salt. Mix well, until evenly distributed.
Season the Cauliflower: Each cauliflower steak has a “pretty” side (the center cut that looks like a tree) and a less-pretty side (the outer edges). I recommend seasoning the less-pretty side first as it browns slightly less. Place the cauliflower steaks (less-pretty side up) on a baking sheet. Use a pasty brush to brush about half of the spice rub across the cauliflower. Set aside.
Sear: Warm a cast iron skillet over medium-high heat with the remaining oil. (Note: the oil should cover the entire base of the skillet; if it doesn’t, add more). Place the cauliflower steaks spice-side-down into the skillet and sear for 3-4 minutes. Carefully brush the unseasoned side of the cauliflower steaks during this time, then flip and sear for an additional 2-3 minutes.
Bake: Remove the pan from the stove and place in the middle rack of the oven (Note: if all of your steaks do not fit, transfer them to the baking sheet instead). Roast for 15 to 20 minutes, until the cauliflower is golden and the central stem is tender.
Make the Chimichurri: In the meantime add the parsley, oregano, garlic, red wine vinegar, salt, and pepper flakes to a medium bowl. Mix well as you slowly pour in the olive oil; set aside.
Serve: Remove the cauliflower from the oven and transfer to a platter or serving plates. Serve warm, topped with chimichurri. This recipe is best enjoyed fresh, but leftovers will keep in the fridge for up to 3 days.