Preheat the oven to 200C/Fan 180C/Gas 6. Put the onions, potatoes, garlic and tomatoes in a large roasting tin and season with sea salt and lots of freshly ground black pepper. Toss everything together lightly and roast for 20 minutes.
While the vegetables are roasting, skin the chorizo and cut the meat into thin slices - 5mm is about right. Put the chicken thighs on a board and carefully slash each one 2 or 3 times with a knife. Season all over with black pepper. Mix the paprika and oregano together and set aside.
Take the roasting tin out of the oven, scatter the chorizo over the veg and turn everything a couple of times. Place the chicken on top of the vegetables and chorizo and sprinkle with the paprika and oregano. Season with a little salt and return to the oven for 20 minutes.
Take the tin out of the oven. Holding one corner carefully with an oven cloth, lift the tin a little so all the juices run to the opposite end, then spoon and drizzle the juices back over the chicken. Tuck the pepper strips loosely around the chicken and vegetables.
Turn the oven up to 220C/Fan 200C/Gas 7. Put the tin back in the oven for another 20 minutes or until the peppers are just softened and the chicken is golden and crisp. As you eat, squeeze the garlic out of the skins and enjoy the deliciously soft and fragrant flesh. Just don't kiss anyone afterwards!