Preheat the oven to 120 degrees Celsius.
Heat the oil in a heavy bottomed pan on a medium heat. Add the beef and cook until browned, then remove from the pan.
Peel and roughly chop the onion and carrots. Add to the pan and cook until starting to soften, then return the beef to the pan along with the flour. Stir well to coat everything in the flour, then gradually add the ale.
Add the rosemary, thyme, bay leaves, Worcestershire sauce, mustard and a teaspoon of salt. Stir well, put a lid on and transfer to the oven for at least 3 hours. Remove from the oven and leave to cool slightly. Turn the oven up to 180 degrees.
Remove the florets from the cauliflower head and cook in boiling water for 5 minutes, then drain.
In another pan, melt the butter over a medium heat and add the flour. Whisk.
Turn off the heat and gradually add the milk, whisking all the time. When you have added all the milk, turn the heat back on and continue whisking until the sauce thickens. Stir through the cheese and cauliflower florets.
To compile your pie, get a large ovenproof dish. Tip the stew into the bottom and spoon the cauliflower cheese evenly on top. Roll out the pastry and place on top, using a fork to crimp the edges and pierce air holes in the middle. Beat the egg and brush over the pastry. Bake in the oven for 40-50 minutes until golden brown.