1. Fill your kettle and boil it. Trim the root and top 3cm off the leek. Halve it and rinse out any grit, then finely slice the leek. Trim the dry ends off the celery stick and then finely slice it.
2. Put a large pan on a medium-high heat for 2 mins, then add ½ tbsp oil and the beef stir-fry strips. Fry for 4-5 mins, stirring a few times, till the beef strips are browned all over. Lift them out of the pan onto a plate and set aside.
3. Add another ½ tbsp oil to the pan and add the leek and celery. Season with salt and pepper, turn the heat down to medium and fry for 5-6 mins. Stir every few mins and the veg should end up glossy but not too browned. If they start to brown, turn the heat down and add a splash of water.
4. While the veg fry, trim and peel the carrots and parsnips, then slice them on the diagonal to make 1cm-thick rounds.
5. Stir the beef back into the pan, then add the sliced carrots and parsnips. Add ½ tbsp horseradish mustard (keep the rest for later) and stir to mix. Drop in the bouquet garni and crumble in the stock cube. Pour in 1 ltr boiling water. Stir a couple of times, then pop a lid on the pan and bring to the boil. Once it’s boiling, turn the heat down and simmer for 10 mins.
6. While the stew simmers, make the dumplings. Tip the flour into a mixing bowl and add 1 tsp baking powder and a pinch of salt and pepper. Whisk with a fork to mix. Pop another ½ tbsp horseradish mustard into a bowl and add 1 tbsp olive oil and 4 tbsp warm water. Stir, then add to the flour and stir again to make a loose dough. If it’s a little dry, add another 1-2 tbsp water and mix.
7. Divide the dough into 4 pieces and shape into dumplings. Take the lid off the stew, taste and add a pinch more salt or pepper if you think it needs it. Add the dumplings to the pan and pop the lid back on. Simmer for another 20 mins, till the dumplings have doubled in size and the veg and beef are tender.
8. Ladle the stew and dumplings into 2 warm bowls, discarding the bouquet garni, and serve.