To make the lamb ragu, pre-heat the oven to 160°c/320°F.
Place the lamb shoulder, along with the quartered onion, garlic cloves, rosemary, half of the red wine and stock in an oven-proof dish then season to taste and cover with foil/a lid.
Place in the oven and allow to roast slowly for 3-4 hours until the lamb is tender and falling off the bone.
In the meantime, place the potatoes on a baking sheet and bake for 45 minutes – 1 hour until a skewer is easily inserted.
Place the cauliflower on a baking sheet and drizzle with a little olive oil. Allow to roast alongside the potatoes until golden brown and cooked through.
Remove the potatoes and cauliflower from the oven and allow to cool slightly.
Puree the cauliflower and set aside.
Halve the potatoes and scoop the flesh into a bowl, using a potato masher/fork, mash until there are no lumps. Add the cauliflower puree and allow to cool to room temperature.
Add the egg yolks and mix well.
Add the flour, 1/2 cup at a time and mix in gently, you don’t want to over-work the dough. I only needed 1.5 cups of flour for my gnocchi but some mixtures might need a bit more. It’s best to make sure your potatoes are quite dry before adding the flour though as the more flour you add, the tougher your gnocchi will be. The dough should be soft and a little tacky, but not overly sticky.
Break tennis ball sized pieces off of the dough and roll into long strips on a floured surface. cut the strip into 1cm gnocchis and place on a tray sprinkled with semolina (the semolina won't absorb into the gnocchi) until you are ready to cook them. If you like, you can also use a fork to make indents in the gnocchi which create little spaces to suck up more of the ragu sauce.
When the lamb is cooked, remove from the oven and shred.
In a saucepan, saute the red onion and sliced garlic until soft and translucent then add the tomatoes, tomato paste, shredded lamb and all the lamb's cooking juices.
Reduce the heat and allow to simmer for 30-45 minutes until the sauce has reduced slightly and the lamb is very tender.
To cook the gnocchi, bring a large pot of salted water to the boil and drop the gnocchi in carefully. When they float to the surface they are ready. Remove them with a slotted spoon and place straight into the ragu.