Pre heat the oven to 200°C 392°F
Peel, halve and thinly slice the onions. Season generously inside the chicken cavity or over the thighs. Pour two tablespoons of oil into a casserole dish. Add the onions. Season generously with salt and pepper.
Place the whole chicken or poussin, breast side down, on top of the onions. Press down fimly, or arrange the thighs snuggled up closely in a single layer. Pour the final two spoons of oil over the top. Position the lid or use foil to an efficient leak free lid. Cook in the middle of the oven for 75-90 mins. If using a poussin: 60 mins.
Serve with mashed potatoes.