Heat the oil in a saucepan. Add garlic and onion and fry until soft and slightly brown. Increase heat, add ground beef and thyme and fry until beef changes color.
Add wine, half the stock, Worcestershire sauce and tomato paste. Simmer for 10 minutes.
Meanwhile, boil the unpeeled potatoes, then peel them. Add boiling milk and mash potatoes while they are still hot. Season with salt and freshly ground pepper and add butter and chives, if using. Preheat the oven to 350 degrees.
Bring the rest of the stock to a boil and thicken it well with roux. Stir it into beef – it should be thick but still juicy. Taste and correct seasoning. Put meat mixture into one large or six individual pie dishes.
Pipe or spread mashed potato mixture over the top. Sprinkle with grated cheeses. Bake for 30 minutes, until top is golden and slightly crispy. Serve with garlic butter and a green salad.