Place 20g plain flour in a large mixing bowl and then add your diced lamb, and toss it to coat the pieces. In a large casserole dish, heat some olive oil and then, in two batches, brown the pieces of meat and then remove and keep to one side. You want to ensure the meat has a nice caramelisation on all sides as this locks in the flavour. Parts of meat may stick slightly to the bottom of the pan, therefore in between each batch use a small glug of red wine to deglaze the pan, scraping all the flavoursome juices and bits of meat into the wine and then pouring this off into a jug (save this for later). Repeat the deglazing process once more after you have browned all your pieces of meat.
To your pan add the lardons, and fry these off until they begin to go crispy. Next add the diced onion and fry until soft, adding a little more oil if necessary. Next add your crushed garlic, carrot and celery and cook for 5 minutes. To your vegetables and lardons, add 10g plain flour and the tomato puree. Cook this for 2 minutes, then add the deglazing meat juices from earlier, 200ml of red wine and 500ml of lamb stock. Add back your browned lamb, a bay leaf, and sprigs of rosemary and thyme. Bring the whole mix up to the boil and then reduce to a simmer, cover with the lid and allow to cook for 1 hour.
After an hour, remove the lid and let the mixture simmer for another 15 mins until the sauce has reduced and thickened. Remove the bay and rosemary sprigs. Preheat your oven to 190C.
Meanwhile, make the crumble topping. In a large mixing bowl add the flour and the diced butter. Using your hands, rub the butter into the flour to create a rough mix similar to breadcrumbs but allow some bigger lumps of butter to remain. Next add your oats, chopped nuts, crumbled cheeses and thyme leaves. Season the whole mixture. In a baking dish, spoon in the lamb stew mixture and then cover fully with the crumble topping. Place in the oven for 30 – 40 minutes until the crumble topping has gone golden brown. Garnish with fresh thyme sprigs.