How to make it
For the salsa: Heat broiler to high. On a roasting pan, arrange tomatillos and jalapeños and roast until skin has black spots, flipping once, 5 to 7 minutes per side; set aside to cool. In a food processor, blitz onion, cilantro, and salt until finely chopped; add tomatillos and jalapeños and blend until smooth. Transfer processor bowl to refrigerator until cool. Return bowl to base, add avocado, and blend until creamy.
For the cauliflower steaks: Heat oven to 375°F. Drizzle cauliflower with oil and season with salt. In a very hot pan coated with oil, over medium-high heat, sear cauliflower, flipping once, 1 minute per side.
Transfer cauliflower to a roasting pan and continue cooking until fork-tender, 15 minutes; serve immediately with salsa.