How to Make It
In a large pot, combine the hot water with the crushed red pepper, garlic and the 1/2 cup of salt and stir until the salt is dissolved. Add the cold water and the capon. Cover and refrigerate for 6 hours.
Preheat the oven to 350°. Drain and rinse the capon, then pat dry. In a medium skillet, cook the chorizo over moderate heat until lightly browned, about 5 minutes. Transfer to a medium bowl. Add the smoked cheddar, bread cubes and stock and stir. Loosely stuff the capon with half of the mixture and transfer the rest to a small baking dish.
Transfer the capon to a roasting pan and tie the legs together with kitchen string. Rub the bird with olive oil and season generously with salt and pepper. Roast the capon for about 2 hours until golden and an instant-read thermometer inserted in the meaty part of the thigh registers 170°; turn the pan occasionally during roasting for even browning. Let the capon rest for 20 minutes.
Meanwhile, bake the stuffing in the baking dish for 20 minutes, until heated through and browned and crusty on top.
In a medium skillet, cook the butter over moderate heat until nutty and lightly browned, about 5 minutes. Add the sorrel and a pinch of salt and cook, stirring until wilted, about 1 minute.
Spoon the stuffing from the cavity of the capon into a serving dish. Carve the capon and transfer to a platter. Spoon the sorrel butter over the capon and serve with the baked stuffing.