Special equipment: butcher's twine
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.
Position an oven rack in the center of the oven and preheat to 475 degrees F.
In a small bowl, stir together 6 tablespoons butter with 1/4 cup of the harissa until smooth and well-combined; set aside. Place the chicken breast-side-up on a wire rack fitted into a rimmed baking sheet. Slide your forefinger gently under the skin of the chicken and loosen it from the breasts and thighs. Using a spoon, stuff the harissa butter underneath the skin of the breasts and thighs and press it so it spreads out evenly. Rub the exterior of the chicken with the remaining 2 tablespoons butter and sprinkle generously with salt and pepper. Stuff the cavity of the bird with the onion and celery and tie the legs together with butcher's twine.
Roast the chicken for 20 minutes, then reduce the heat to 400 degrees F and continue to roast until the skin is crispy and golden brown and a thermometer inserted into the thigh (avoiding bone) reads 160 degrees F, about 45 minutes longer. Remove from the oven and immediately brush all over with the remaining harissa. Set the chicken aside to rest for at least 10 minutes, then serve.
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