Heat 2 tablespoons of oil in a large, shallow casserole. Brown the garlic, ginger, onions, and pepper for 3 minutes until soft and fragrant. Set aside. Toss the meat in the flour, add 1 tablespoon of oil to the pan, then brown the meat in batches, adding the final tablespoon of oil if necessary. This should take about 5 minutes to brown each batch properly.
Add the five-spice and star anise (if using) to the pan, add the ginger mixture, and fry for 1 minute until the spices are fragrant. Add the sugar, then the meat, and stir until everything is well combined. Keep the heat high, then pour in the wine or sherry, scraping up any meat bits that have stuck to the bottom. Heat the oven to 150C/fan 130C/gas 2.
Add the soy sauce and broth (it won’t quite cover the meat), bring to a boil, then cover tightly, transfer to the oven, and cook for 1 1/2 to 2 hours, stirring the meat halfway. The meat should be very tender and the connective bits should have melted away. Season with more soy sauce. This can now be refrigerated or frozen for up to 1 month.
Put the cooked bok choy in the pan, then bring to the table with steamed basmati rice and eat immediately.