Peruvian chicken is slightly citrusy from the lemon, tangy from the vinegar, earthy from the oregano, and spicy––from the cumin, paprika, and garlic. The green sauce is similarly multidimensional green sauce made of jalapeños, cilantro, garlic, lime, and vinegar are flavors that were meant to go with roast chicken.
In a bowl, combine the garlic, cumin, paprika, oregano, salt, pepper, olive oil, vinegar, and both the zest and juice of the lemon.
Rinse the chicken, remove any giblets, and pat the bird dry with paper towels. Place the chicken over the sliced onions in an enameled baking dish, and use your fingers to gently spread the spice mixture under the skin, on the outside of the chicken, and inside the cavity, until it is thoroughly seasoned. Tie the legs together with kitchen string. Cover and refrigerate for 4-24 hours.
When you’re ready to roast the chicken, remove it from the refrigerator, and allow it to come back up to close to room temperature (it shouldn’t be ice cold from the fridge). Meanwhile, preheat your oven to 425 degrees F.
Roast the bird in the oven for 15 minutes and then baste it with any juices in the roasting pan. Reduce the heat to 375ºF (190°C) and continue to roast, for 45 to 60 more minutes, or until the juices of the chicken run clear when the thigh is pierced with a fork and an instant-read thermometer inserted into the thickest part of the thigh reads 165ºF (74°C). You can baste it a couple times as it continues to roast. Allow the chicken to rest for a few minutes after you remove it from the oven.
While the chicken is roasting, prepare the sauce. Combine jalapeños, cilantro, garlic, mayonnaise, lime juice, and vinegar in the jar of a blender. Blend on high speed, scraping down as necessary, until smooth. With the blender running, slowly drizzle in the olive oil. Season to taste with salt and pepper. Transfer to a sealed container and refrigerate until ready to use.