Heat the oil in a large saucepan or deep sauté pan. Tip in the onions, bay leaves and thyme sprigs, and fry over a medium heat for about 8 mins, stirring often, until the onions are turning golden. Raise the heat, add the steak and stir-fry briefly until it starts to lose its raw, red colour. Pour in the wine, stir to deglaze the brown sticky bits from the bottom of the pan, and let it bubble briefly. Lower the heat, sprinkle in the flour and mustard powder, and stir for 1 min. The meat should now be coated in a thick, rich sauce.
Mix in the tomatoes, stirring to break them down. Stir in the stock and bring to the boil. Tip in the carrots, squash and mushrooms, lower the heat, cover with a lid and leave to simmer gently for 1 hr 40 mins, stirring occasionally. Uncover and cook for a further 20 mins, still on a gentle simmer, until the meat is very tender. Season with pepper.
When the stew is nearly cooked, heat oven to 190C/170C fan/gas 5. Put a 2.25-litre casserole dish in to heat it up. Meanwhile, make the dumplings. Put the flour, mustard powder, some pepper and a pinch of salt in a bowl, then stir in the spring onions and parsley. Mix the oil and buttermilk together and gently stir into the flour. Add a drop or two of cold water, if needed, to pick up any dry bits on the bottom of the bowl, and stir to make a soft and slightly sticky dough. Be as light-handed as you can, as overmixing or overhandling will toughen the dumplings. Cut the dough into 8 pieces and very lightly shape each into a small, rough ball.
Carefully transfer the stew to the hot casserole dish and remove the bay leaves and thyme sprigs. Sit the dumplings on top and press them down into the gravy to very slightly submerge. Put the dish on a baking sheet and cook for about 20 mins until the dumplings have risen and are golden on top. Serve with a light scattering of thyme leaves.