Preheat the oven to 400 degrees F. In a large baking dish or cast-iron skillet, arrange the potatoes, onion and thyme. Season the vegetables with salt and pepper and add in the stock or white wine. Arrange the chicken over the vegetables and season liberally with salt and pepper. Drizzle the chicken with about 3 tablespoons of the oil.
Place the pan into the oven and roast until the potatoes are tender and chicken is cooked through, 45 to 50 minutes.
As soon as the pan goes into the oven, tear off a small square of aluminum foil. Cut about 1/2 inch off from the top of the head of garlic to expose all of the cloves. Place the garlic cut-side up on the piece of foil, drizzle it with the remaining 1 tablespoon of oil, and season it with salt and pepper. Close up the foil around the garlic and roast it in the oven until the cloves are tender and golden brown, 35 to 40 minutes.
When everything is ready, transfer the chicken to a serving plate. Squeeze the garlic out of its head into a small bowl and mash it into a paste. Add the garlic paste to the pan with the potatoes and toss to coat. Serve the chicken with the potatoes and some Dijon mustard.
If you want to get a bit of green into this meal, toss 1 1/2 cups baby spinach in with the hot potatoes before serving (the residual heat will wilt it down).