Fill your kettle and boil it. Tip the dried red lentils into a medium-sized pan and crumble in the vegetable stock cube. Pour in 600ml boiling water and stir to mix and dissolve the stock. Pop a lid on the pan, bring to the boil, then turn the heat down to low and simmer for 15 mins till the lentils are thick and tender. Stir every so often.
While the lentils cook, put a large, deep frying pan or wok on a medium heat. Tip in the walnut halves and dry fry, shaking the pan every so often, for 3-4 mins till browned and nutty. Tip into a small bowl and set aside.
Peel and finely chop the onion. Trim the dry ends off the celery and dice it to match the onion. Trim and peel the carrot, then dice it to match the other veg. Put the frying pan back on a medium heat and warm for 2 mins. Add 1 tbsp oil and the chopped veg. Season with a pinch of salt and pepper and fry, stirring occasionally, for 8 mins till glossy and starting to soften.
While the veg fry, slice the chestnut mushrooms. After 8 mins, add the mushrooms to the pan and stir well. Add another small pinch of salt and pepper and fry for 8 mins more, stirring, till the mushrooms are soft and juicy.
While the mushrooms fry, heat your oven to 190°C/Fan 170°C/Gas 5. Peel and crush or grate the garlic. Pick the leaves off the rosemary and parsley sprigs. Heap them up on your chopping board and finely chop them together.
The lentils should be cooked by now, so make sure they are drained and set aside in a sieve. Stir the garlic, rosemary and parsley into the veg with 1 tsp dried oregano and the baby leaf spinach. Cook and stir for 2 mins, then take off the heat.
Finely chop the toasted walnuts and add them to the pan. Crumble in the Perl Las blue cheese (including the rind). Add the breadcrumbs and 1 tbsp Worcester sauce, then tip in the cooked red lentils. Stir well to mix, taste a little of the mix and add a pinch more salt or pepper if you think it needs it.
Beat the eggs together in a small bowl, then tip them into the pan with the veg and lentils. Stir everything together to mix well. Spoon the mushroom, walnut and spinach mix into a 2 ltr ovenproof dish.
Slide the roast into the oven and bake for 45 mins till the top of the roast is a dark brown and crisp. When it’s ready, take the roast out of the oven and let it cool in the dish for 10 mins before serving with the Shiitake & Porcini Mushroom Gravy poured over the top.