Preheat oven to 350F/180C.
Cut red onion into wedges, halve the larger potatoes.
Toss potatoes, onion, leek, and garlic with oil in a large roasting pan. Top with chicken, season with a little salt and plenty of pepper. Roast 40 minutes.
Remove from oven. Stir in kale, drizzle stock over everything. Return to oven for 5-10 minutes.
For an easy gravy, add 1-2 tbsp sherry vinegar to cooking juices.