Preheat the oven to 160°C/°F/Gas Mark 3.
Add the beef to a hot, deep casserole dish with a dash of oil and sprinkle over the flour. Give the meat a good stir until the flour disappears from sight and the meat is very well-browned.
Add the remaining ingredients needed for the filling; the mushrooms, garlic, onions, carrots, celery, ale, beef stock, thyme and mustard. Again, give everything a good stir so they are evenly distributed and season well.
Pop the lid on and place the dish in the preheated oven for 3 hours, occasionally stirring to make sure the bottom isn’t catching, or adding a little water if you think the mixture is becoming dry.
While the stew is cooking, make your mash. Simply empty the celeriac and potato into a large saucepan and fill with cold water.
Bring to the boil and simmer for 15–20 minutes, until the vegetables are completely tender. Drain and mash, stirring in the milk and knob of butter. Season to taste.
Remove the casserole dish from the oven and allow your pie filling to cool. Transfer to a 1.5 litre, ovenproof dish.
Preheat the oven to 180°C/350°F/Gas Mark 4.
Bake for 30 minutes until the top is crusty and golden brown and the filling is bubbling below. Garnish with a little fresh thyme before serving