Preheat oven to 400 degrees F. Core and quarter to sixth the apples, depending on the size of the apples. Cut the onions into wedges of a similar size to the apple wedges.

In a 9x13 to 10x15 glass pan, toss together the apples, leeks, half the oil, rosemary, 1/2 tsp salt or so, and a few grinds of black pepper. Make sure to coat everything in the oil. Oil and season the chicken and nestle, skin-side-up, among the vegetables.

Roast until the chicken is cooked through and the apples and onions are tender, 40-45 minutes.