Tear open the New Mexico and guajillo chiles and discard the seeds and stems. (You may leave the seeds in if you prefer a spicy harissa.) Tear the chiles into large pieces and place them in a medium bowl. Pour 2 cups boiling water over the chiles and let stand until softened, about 20 minutes.
Put the coriander and cumin seeds in a small skillet and cook, swirling often, over medium heat, until the seeds are fragrant and lightly toasted, 2 to 3 minutes. Transfer the spices to a blender and add the oil, garlic, lemon juice and 2 teaspoons kosher salt, then puree until smooth. Using tongs, add the chiles to the blender along with 1/2 cup of their soaking liquid (discard the rest) and puree until very smooth, about 2 minutes on high. Season the harissa with pepper and set aside.
Preheat the oven to 450 degrees F.
In a large bowl, toss the cauliflower with 3/4 cup of the harissa until each floret is evenly coated. Spread the cauliflower onto a rimmed baking sheet and drizzle with a little more vegetable oil. Roast, tossing halfway through, until the cauliflower is tender and the harissa has begun to caramelize, about 40 minutes.
Transfer the cauliflower to a serving platter and sprinkle with the cilantro. Serve with the remaining 1/4 cup harissa on the side for dipping.
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