Preheat the oven to 425°. In a medium bowl, toss the onions, olive oil and the 2 teaspoons of mint. Season with salt and black pepper.
In a small bowl, mash the butter with the garlic, scallions, Aleppo pepper, lemon zest and the remaining 1 1/2 tablespoons of mint; season with salt. Rub the seasoned butter evenly under the skin of the chicken. Tie the legs together with kitchen string and set the chicken in a medium roasting pan.
Season the chicken with salt and black pepper and roast for 20 minutes. Add the onions to the pan, reduce the oven to 375° and roast for 20 minutes longer. Squeeze the lemon wedges over the chicken and add to the pan. Roast for 35 minutes longer, basting, until an instant-read thermometer inserted in the thigh registers 165°. Transfer the chicken to a surface. Let rest for 10 minutes.
Return the pan with the onions and lemon to the oven; roast for 10 minutes. Remove the string and carve the chicken. Serve with the onions, lemon wedges and pan juices.