Heat a large pan and cook the onion and lardons together for 5 mins until golden. Scoop out with a slotted spoon and set aside. Add the oil to the pan, dust the beef with the flour, then evenly brown over a high heat.
Add the parsnips, ale, stock, jelly, thyme and lardon mixture to the pan. Bring to the boil, then cover and simmer for 1½ hrs until the meat is tender.
Generously butter a 1.5-litre pudding basin. To make the pastry, mix together the flour, mustard powder, suet and ½ tsp table salt. Add enough cold water, about 150ml, to make a soft dough. Remove one-quarter of the dough and set to one side. On a heavily floured surface, roll out the remaining dough to make a large round, big enough to line the basin.
Carefully lay the pastry in the basin (aim to have 1cm of pastry overhanging the rim), then press the edges of the join together to seal. Roll out the remaining one-quarter into a circle big enough to cover the top.
Pour off the cooking liquid from the filling into a small pan and set aside. Discard the thyme stalks. Spoon the filling into the lined basin and pour over 100ml of the cooking liquid. Fold over the overhanging pastry and brush with water. Place the lid on top, pressing firmly around the edges to seal.
Butter a sheet of baking parchment, fold in a large pleat and lay, butter-side down, on top of the pudding. Cover with a pleated layer of foil and finally tie with string, making a loop for the handle so you can lift the pudding easily.
Sit a small trivet or a large cookie cutter in the bottom of a deep saucepan that’s big enough to take the basin easily. Half-fill the pan with water and bring to the boil. Lower in the pudding, cover and simmer for 2 hrs, topping up with boiling water when necessary.
Reheat the cooking liquid, bubbling it down a little so it reduces into a tasty gravy. Carefully lift out the pudding. Run a knife around the rim, then turn out and serve with gravy and greens, if you like.