MAKE THE HERB SAUCE: In a blender or food processor, pulse the parsley, cilantro, mint, green onion, garlic, lemon juice and olive oil until completely smooth. Set aside.
Heat the remaining 1 tablespoon olive oil in a large cast-iron skillet over medium-high heat. Working in batches, sear the cauliflower steaks until they are golden brown, 3 to 4 minutes per side. The cauliflower should be easily pierced with a fork but not so tender that it falls apart.