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1
Remove the leaves and trim the stem of the cauliflower, leaving the core intact. Place the cauliflower core-side down on a work surface. Starting in the center of the head, slice from top to bottom into four 1-inch (2.5 cm) steaks. Place steaks on a rimmed baking sheet and any florets that break loose in a bowl. Drizzle the oil over the steaks and florets and generously season with kosher salt and pepper.
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2
Prepare a charcoal grill for two-zone cooking and build a medium-high fire, or heat a gas grill to high. Carefully wipe the preheated grates with a lightly oiled paper towel. Using a grill brush, scrape the grill grates clean, then carefully wipe with a lightly oiled towel again. Allow a small cast iron skillet or grill basket to heat for 5 minutes before cooking.
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3
To make the harissa, blister the tomatillos, onion, chiles, and garlic in the preheated small cast-iron skillet or grill basket over direct heat, until charred and softened on all sides, 4 to 5 minutes for the tomatillos and chiles, a bit longer for the onion and garlic. Set aside to cool.
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4
Stem and seed the chiles, peel the garlic, and place them both in a food processor. Add the tomatillos, cilantro, parsley, arugula and spinach, vinegar, lemon zest, and olive oil and puree until smooth. Season with salt and pepper.
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5
Grill the cauliflower steaks (in a grill basket, if desired) over direct heat, rotating them around the fire as needed to prevent them from blackening before they're cooked, until deeply charred on the exterior and just tender at the core, 8 to 10 minutes per side. Grill any loose florets in a grill basket, tossing often, until browned and crispy, 5 to 7 minutes.
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6
Serve the warm steaks on a pool of harissa and garnish with the crispy bits of florets and a sprinkle of flaky salt.
