Set oven temperature to 220°C (200°F, Gas 7). Place the steak in a sizable, heavy skillet and cook it on high heat for five to six minutes, or until it turns brown. Coat the steak with low-calorie cooking spray.
Stir in the shallots, mushrooms, celeriac, thyme, stock, Guinness, and gravy granules. After bringing to a boil, lower the heat to medium and simmer the sauce until it thickens, about 6 to 8 minutes.
Take off the heat and transfer the mixture into four 12cm-long, deep oval pie dishes.
To fit the pie dishes, roll out the pastry and cut four lids. Use a fork to press down on the pastry lids after brushing the dish rims with water. Apply beaten egg to each lid. Bake until golden, 15-20 minutes.
When the potatoes are ready, boil them for 12 to 15 minutes, adding sprouts for the final five minutes. After draining, put it back in the pan with the stock. Mix thoroughly and serve right away.