Preheat the oven to 400 degrees F (200 degrees C). Line a baking sheet with parchment paper.
Combine parsley, olive oil, red wine vinegar, garlic, red pepper flakes, oregano, and salt in a bowl. Set aside.
Remove the tough bottom leaves from each cauliflower, taking care not to remove the stem. Place the cauliflower heads onto a cutting board with the stem side facing upwards; cut vertically on both sides of the stem to remove the loose florets. Cut each cauliflower vertically through the stem into 2 even "steaks", 1 to 1 ½ inches thick; remove any remaining small leaves from each steak. Reserve remaining cauliflower for another use.
Place cauliflower steaks on the baking sheet. Brush both sides with olive oil and sprinkle with garlic salt.
Roast in the preheated oven until the edges are golden brown and the cauliflower is tender, 25 to 30 minutes, flipping halfway through. Place cauliflower on a serving platter and spoon chimichurri sauce over top.
