Preheat the oven to 170°C fan bake. Toss the potatoes, onion, tomatoes, wine and garlic in a large bowl and season. Spread evenly in a roasting dish and bake for 30 minutes.
Season the chicken cavity and squeeze in the lemon, then add the squeezed halves and herbs. Tie the legs with kitchen string and tuck the wings under. Place on top of the potatoes. Brush with oil and season well. Roast for 1½ hours, or until fully cooked.
Dressing: Combine all the ingredients and season well.
To serve: Spoon over some dressing, serving the rest separately.