Heat your oven to 200°C/Fan 190°C/Gas 6. Peel and finely chop the onion and garlic.
Warm 1 tbsp oil in a frying pan. Add the onion and garlic. Cook and stir over a medium heat for 3-4 mins till just softened. Take off the heat.
Grate the Parmesan and zest the lemon. Thinly slice the courgettes into rounds. Layer the courgettes, onion, Parmesan, lemon zest and breadcrumbs in a small ovenproof dish till you have used them all up - finish with a layer of Parmesan and breadcrumbs. Put to one side.
Find the white seam on the underside of the chicken legs that divide the thigh from the drumstick. Cut through it. Fine the bone joint with your fingertips. Slide your knife tip in (taking your fingers out of harm’s way) and slice through the joint.
Place the chicken thighs and drumsticks on a baking tray and rub with 1 tbsp oil and plenty of seasoning. Pop both the chicken and the gratin into the oven for 35-40 mins till the chicken is cooked through and the gratin is brown and bubbling.
Make a dressing for the chicken while it cooks. Pick the leaves from the oregano stalks and finely chop them. Juice the lemon and add it to the oregano with 1 tbsp oil. Season and whisk together.
Serve scoops of the gratin along with the crispy chicken and a handful of the spinach leaves. Spoon over the oregano dressing and tuck in.