Preheat the oven to 350°. In a small saucepan, melt the butter with the maple syrup and 2 teaspoons of coarsely ground pepper. Remove the glaze from the heat.
On a large heavy-duty rimmed baking sheet, toss the sliced onions and potatoes with the olive oil. Season the onions with salt and pepper and spread in an even layer. Season the chicken inside and out with salt, then arrange on top of the onions.
Roast the chicken for 45 minutes. Brush the chicken all over with half of the maple glaze and continue roasting for 1 hour longer. Increase the oven temperature to 425°. Generously brush the chicken with the remaining glaze and roast for 15 minutes longer, or until the cavity juices run clear and the chickens are richly browned.
Remove from heat and allow to rest for at least 10 minutes.