Meanwhile, heat 1 tablespoon of the oil in a 14- to 16-inch paella pan or skillet. Season the chicken with salt and pepper; cook over moderate heat until golden, 10 minutes. Transfer to a plate. Add the onion and the remaining 2 tablespoons of oil to the pan; cook over low heat until softened. Add the chorizo, saffron and cayenne; cook over moderately high heat, stirring, for 2 minutes. Add the rice and 1 1/2 teaspoons of salt; cook, stirring, for 2 minutes. Stir in the stock, tomatoes, chard, leek and squash.