Preheat the oven to 180°C/350°F/Gas 4. Heat a casserole dish, add the oil, then tip in the bacon and cook for 3-4 minutes until sizzling. Increase the heat and add the beef and cook for another 6-8 minutes until browned. Remove all the meat with a slotted spoon and add the onions, carrots and celeriac. Sauté for about 5 minutes until starting to colour.
Sprinkle the flour into the vegetable mixture and stir to combine. Return the meat, along with the thyme and pour in the wine, stock and Worcestershire sauce. Season to taste and then cover and braise in the oven for 1 hour 15 minutes without stirring.
After the beef casserole has been in for an hour, make the cobbler topping. Sieve the self-raising flour and mustard into a large bowl and add the herbs with a good pinch of salt and pepper. Rub in the butter until the mixture resembles fine breadcrumbs. Make a well in the centre and add the buttermilk, then bring gently together to make a soft dough. Roll out on a lightly floured board to about ¼ inch thickness and then stamp out 3 inch rounds using a straight-sided pastry cutter. Re-roll the trimmings and cut out more rounds until all the dough had been used up.
After 1 hour and 15 minutes remove the casserole from the oven and take off the lid. Arrange the rounds of dough around the edge in an overlapping layer. Brush the tops with the beaten egg and bake for another 35-40 minutes until the cobbler topping is well-risen and golden brown. Gently split a little of the cobbler topping to ensure that they are completely cooked through, then serve straight to the table.