Season both sides of the lamb slices with salt and pepper and set aside.
Add avocado oil to a large dutch oven or rondeau pan placed over medium high heat. When the oil starts to smoke, add the lamb and sear all sides until golden brown. Remove the seared lamb and set aside.
Add the chopped onion and butter to the pan and reduce heat to medium. Caramelize the onions till golden brown (about 10 minutes) then add the garlic, flour and tomato paste and whisk together. Cook for an additional one minute before whisking in beer and half of the beef stock.
Add the seared lamb back to the pan, cover and reduce heat to low. Allow the lamb to braise for 90 minutes.
Remove lamb from the stew, and set aside. Whisk the cornstarch into the remaining beef stock and add it along with the root vegetables, rosemary, and thyme to the pan. Cover and allow to simmer for 30 minutes.
While vegetables are simmering, pull the lamb into chunks, removing any fat or silverskin and discard. Add the lamb chunks (and any juice collected) back into the stew. Season to taste with salt and pepper and simmer for 10 minutes.
Remove the rosemary and thyme sprigs, garnish the stew with parsley, and serve hot.