Rinse duck breast fillets and pat dry. Season with salt and pepper. Cut pineapple into small pieces. Heat a dry pan and place duck breast fillets skin side down and brown. Turn and cook other side. Place duck breast fillets in an ovenproof dish and bake in a preheated oven at 180°C (fan: 160°C, gas mark 2-3) (approximately 350°F) and cook for 20 minutes.
Pour out fat from pan. Add grappa to taste to make a gravy. Pour chicken stock into pan and bring to a boil. Add pickled peppercorns. Mix cornstarch with a little cold water and stir into sauce to thicken. Stir for 1-2 minutes. Add pineapple and season with salt, lemon juice and sugar to taste.
Remove duck breast fillets from oven and let rest for 5 minutes. Stir gravy into sauce. Cut duck breast and serve with sauce.