If you want spicy harissa, include the chiles' seeds; if not, slit them open to remove and discard the seeds. Tear the chiles into large pieces and place them in a medium bowl. Carefully pour 4 cups boiling water over chiles and set aside until softened, about 20 minutes.
Meanwhile, place the coriander and cumin seeds in a small skillet and cook over medium heat, swirling the pan often, until the spices are fragrant and lightly toasted, 2 to 3 minutes. Transfer the spices into a blender, add the oil, salt, garlic and lemon juice, and puree until smooth. Drain the chiles, reserving 1 cup of the soaking liquid (discard remaining liquid). Add the chiles and reserved liquid to the blender and puree on high until very smooth, about 2 minutes. Season the harissa with pepper and transfer to an airtight container. Refrigerate for up to 2 weeks.
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