Butterfly each shrimp by cutting along the back, through the shell, to remove the black vein. Rinse, then spread it wide open, leaving the shell on. Sprinkle with salt and the freshly ground black pepper. Set aside for at least 1/2 hour.
In a large, heavy pan, with a tight-fitting lid, heat the olive oil over medium-high heat. Season the chicken thighs well with salt and pepper, then add to the hot oil, skin-side down. Cook for about 2 minutes, or until the skin just begins to brown. Add the chorizo slices and toss with the chicken. Cook for 1 minute. Add the shallots, the garlic, and the string beans. Mix well. Add the rice, stirring to incorporate well and to coat the rice thoroughly with oil. Add the stock and the saffron threads. Stir well. Bring the mixture to a boil, reduce the heat, cover, and simmer for 15 minutes.
Embed the shrimp in the top part of the rice, then scatter the snails and the pimento strips around the shrimp, all in a decorative pattern. Cover and cook for 10 minutes more, or until the liquid has been absorbed and the rice is just cooked. Remove from oven, and cover with a damp tea towel for 10 minutes. Serve at the table out of the pan.