For the Basque chicken, put the red onion, peppers, and garlic in a single layer on a shallow roasting tin. Season the chicken pieces all over with salt, pepper, piment dâ€™Espelette and the crushed red pepper flakes and add to the tin. Pour the sherry and lemon juice over the chicken. Cover the pan with cling film and leave to marinate in the fridge for at least 6 hours.
When the chicken is almost ready to cook preheat the oven to 160C/140C Fan/Gas 3 and put a rack in the lower third of the oven
Remove the chicken from the marinade. Strain the liquid and reserve, reserve the vegetables also. Pat the chicken dry and season it with salt and pepper once more. Put a casserole dish over medium-high heat and add olive oil to coat the base of the dish. Add the chorizo, cook for about 3 minutes and then remove and set aside. In the same pot, sear the chicken, skin-side down first, until golden-brown, then remove and set aside.
Add the vegetables from the marinade to the same pan and cook until tender, this should take about 5 to 6 minutes. Pour the reserved marinade liquid into the pot and cook over a high heat until the volume of liquid has reduced by half. Return the chicken and the chorizo to the pot and stir in the stock, and tomatoes. Bring to a simmer, and transfer to the oven, uncovered. Braise until the chicken legs are tender and the chicken is fully cooked through, this should take about 45 minutes.
Meanwhile prepare the salad. Peel the outer layer of the artichokes, trim the stem and scoop away the inside. Place the artichokes in water with a little lemon juice to stop them going brown.
In a small bowl, whisk together the olive oil, sherry vinegar, piment dâ€™Espelette and garlic. Season with salt. Thinly slice the artichokes, preferably on a mandolin, and add the whisked oil. Toss to combine and season with more salt if necessary.
In a large salad bowl, toss together the frisÃ©e leaves, bell peppers, spring onions, red onion and parsley. Serve the Basque chicken with the salad alongside.