Preheat the oven to 190°C fan (400°F), Gas Mark 6.
Put the potatoes into a shallow casserole 30cm (12in) across, or into a roasting tin in which all the vegetables can lie in a single layer. Add the onions, garlic cloves and thyme. Finely grate the zest of 1 lemon over this and squeeze on the juice of half of it. Cut the other lemon into fine slices (flick out any seeds you see).
Add 2 tablespoons of the extra virgin olive oil to the vegetables and season them. Toss the lemon slices in and turn everything over with your hands.
Put the chicken thighs on top, skin side up, and brush them with the remaining olive oil. Season them, too. Make sure no lemon slices are sticking out, as they will burn quickly; they should be tucked under the chicken.
Roast in the oven for 30 minutes. Retrieve some of the lemon slices and put them on top of the chicken, so they can turn golden in the last bit of cooking time. Scatter the capers over and return to the oven for a final 10 minutes.