Pre heat oven to 160˚C / 320˚F (fan bake).
Lightly coat washed, peeled and chopped sunchokes in olive oil and spread over a baking tray. Add lightly oiled garlic cloves – with skins on (remove once cooked). Add thyme and the leaves from the rosemary sprigs. Roast for 35-45 minutes, until sunchokes are soft and golden.
In a large saucepan, heat a tablespoon of oil then add chopped onion, cook till golden, add chopped leaks and cook till soft. Add vegetable stock and oat milk, roasted sunchokes, garlic and herbs. Bring to the boil then reduce to a medium heat and cook for another 10 minutes.
Once cooled slightly, blend the soup in batches till smooth.
In a frying pan, add a small amount of olive oil – just enough to coat the pan, and lightly pan fry the sage leaves till crispy. Transfer to a paper towel to dry before serving with the soup along with fresh thyme, olive oil and a squeeze of lemon. Salt if desired.