Preheat the oven to 220°C/425°F/Gas 7. To make the chips, place the potatoes in a large freezer bag with the olive oil and garlic. Season to taste, then seal the bag and mix until all of the potatoes are evenly coated. Tip on to a large non-stick baking sheet and roast for 25-30 minutes until they are cooked through and crisp, turning occasionally to ensure that they cook evenly.
When you’re ready to cook the steaks, heat a large heavy-based frying pan. Season the steaks generously with the pepper. Add the oil and a knob of the butter to the pan and as soon as the butter stops foaming, add the steaks. Cook for 2-4 minutes on each side, depending on how rare you like your steak. Transfer the steaks to a warm plate, season with salt and set aside in a warm place to rest for 5 minutes.
Add the rest of the butter to the frying pan and then tip in the shallot and sauté for 2-3 minutes until softened but not coloured. Increase the heat, add the mushrooms to the pan and season to taste, then sauté for another 2-3 minutes until tender.
Sprinkle over the brandy – it will flame up for about 5-10 seconds and then subside when the alcohol flame burns off. Continue to simmer until all of the liquid has completely reduced leaving no more than about a tablespoon or two are left in the pan. Add white wine vinegar and allow to bubble down, then pour in the wine and simmer until reduced by half, stirring occasionally.
Stir the stock into the pan with the Worcestershire sauce, sugar and cream. Bring to the boil, then reduce the heat and simmer for 10-15 minutes or until thickened and reduced to a sauce consistency which will coat the back of a wooden spoon, stirring occasionally. Stir in the parsley and enough lemon juice to taste. Season to taste.
Transfer the steaks to warmed plates and pile the chips alongside. Drizzle over the sauce and serve at once with a separate bowl of lightly dressed watercress salad.