Place carrots, celery roots, Jerusalem artichokes, and potatoes in a large baking pan. Season with salt, pepper, olive oil, and thyme and toss to coat.
Season lamb shoulder with salt and pepper. Make small cuts in the meat and place garlic inside. Spread with olive oil (preferably by hand).
Place lamb on top of vegetables and bake in a pre-heated oven at 190℃/ 375°F for 30minutes.
After 30 minutes, turn the meat over and bake for an additional 30 minutes. Wrap with parchment paper and aluminum foil and bake for an additional 90 minutes at 160℃/ 320°F.