Drizzle the oil into the bottom of a large roasting tin. Put chicken thighs into the tin, turning to coat the skin in oil, then turn them skin-side up. Add in the chorizo, the potatoes and the onions in (you may need two tins, if yours are not big enough to hold everything in a single layer). Sprinkle over the dried thyme (or oregano) and the lemon zest. Squeeze over the lemon juice and season everything with salt and freshly ground black pepper.
Cook in the oven for 45 minutes to 1 hour, or until the chicken is cooked through and the potatoes are crisp on the outside and tender on the inside. Serve with a crisp green salad.