Boil the pearl barley in plenty of water, until cooked al dente (around 20 minutes). Drain, and set aside.
Meanwhile, heat a dash of oil in a large, deep frying pan, and add the diced carrot and onion. Cook over a medium heat for about 5 minutes, then add the mushrooms and garlic. Cook for 5 more minutes.
Add the fresh rosemary and thyme to the vegetables, along with the bay leaf, tomato puree, red lentils, butter beans, and veggie stock. Mix to combine, and bring to a gentle simmer. Cook until the lentils are fairly soft, around 15 minutes, stirring occasionally.
While the cassoulet simmers, prepare the dumplings. Add the vegetable suet, flour, dried parsley, salt and pepper to a bowl, and mix to combine. Add the water a little dash at a time, mixing with a spoon until you end up with a slightly sticky dough. I ended up using around 6tbsp water in total.
When the lentils are nearly cooked, add the cooked pearl barley to the pan, and mix the cassoulet thoroughly. If the mixture seems a little dry, just add an extra splash of water (the dumplings need some steam to cook, so you need at least a small amount of liquid). Dollop the dumpling mixture on top - I ended up with 8 dumplings in total. Add a lid to the pan, and cook over a fairly low heat for another 10-15 minutes, until the dumplings are fairly firm, and piping hot inside. For a slightly crispier dumpling, you can also pop the pan under the grill (broiler) for a few minutes.
Remove any large stalks of rosemary and thyme, and serve hot.