Combine the mayonnaise, onion, garlic, curry powder, lime zest and juice in a food processor and process until smooth. Pour the mixture into a large bowl, add the turkey breast and cilantro; turn to coat the entire turkey in the marinade. Cover and refrigerate for at least 2 hours and up to 8 hours.
Preheat the oven to 425 degrees F.
Remove the turkey from the marinade and arrange on a baking rack set in a small roasting pan. Season well with salt and pepper and roast in the oven until lightly golden brown, about 20 minutes. Reduce the heat to 350 degrees F and continue roasting until an instant-read thermometer inserted into the center of the breast registers 160 degrees F, about 1 hour longer. Remove to a cutting board, loosely tent with foil and let rest 15 minutes before slicing and serving.