Marinate chicken thighs, breasts, and drumsticks in olive oil, lemon juice, and salt for at least 1 hour and up to overnight.
Preheat the oven to 375 degrees F (190 degrees C).
Mix onions, sumac, and pepper into the chicken. Pour mixture into a nonstick baking pan. Cover with aluminum foil.
Bake in the preheated oven until an instant-read thermometer inserted near the bone reads 165 degrees F (74 degrees C), about 45 minutes. Remove foil halfway through to give the chicken a nice golden color and crunch.