For the chimichurri, combine oil, lime juice and vinegar in a liquid measuring cup.
Process cilantro, parsley, onion, oregano, garlic, serrano, salt, and zest in a food processor until minced. With processor running, stream in oil mixture until combined; season with salt and pepper. Chill until ready to serve.
Cauliflower Steaks
Trim leaves from cauliflower heads. Place heads steam side down and cut, top to bottom, into 1-inch thick slices, or “steaks" (you should get 4–6 total steaks). Arrange steaks on a sheet pan, brush both sides with oil, and season with salt and pepper.
Coat a large cast-iron skillet with nonstick spray, heat skillet over medium-high for 5 minutes. Add 2 steaks to skillet, place an additional cast-iron skillet or weighted pan on top of steaks, and cook until charred, 3–4 minutes. Flip steaks and cook another 2–3 minutes until charred. Transfer to a serving platter and repeat with remaining steaks.
Serve cauliflower steaks with chimichurri sauce.