Remove the pigeon legs from the body by cutting through the thigh joint. Place the legs on a tray, sprinkle with ½ teaspoon salt, the garlic and herbs, then cover and refrigerate for 1 hour. Remove the breast fillets from the bone and refrigerate until required.
Preheat the oven to 150°C (300°F/Gas 2). Remove the legs from the fridge, rinse off the salt and pat dry with paper towel. Place in a snug-fitting roasting tray and pour in enough oil to cover. Bake for 1 hour, then remove from the oven and allow to cool in the oil. Once cool, pick the meat from the bones, discarding the skin and bones. Season the meat with the mustard, salt and pepper. Press the meat into a 10 cm (4 inch) square container and refrigerate for 3–4 hours or until chilled.
When ready to serve, preheat the oven to 140°C (275°F/Gas 1). Turn the chilled leg meat out and cut into 2.5 cm (1 inch) cubes. You will need 18 cubes. Place on a baking tray lined with baking paper, drizzle with oil and warm in the oven for 5 minutes. Using a microplane grater, grate the pistachios. Remove the meat from the oven and coat with the grated pistachio. Set aside and keep warm.
Meanwhile, season the breast fillets with salt. Heat 2 teaspoons of oil in an ovenproof frying pan over medium heat and sear the breasts until golden on both sides, then roast for 3 minutes — they should be quite pink in the middle. Rest in a warm place until ready to serve. Just before serving, carve each breast fillet in half.