Heat olive oil in a deep pan and add chopped onion and celery, cook for 10 minutes over low heat until softened, add garlic and cook for 1 minute longer, then add lentils and toast them while stirring for 1 minute.
Add the stock, sliced kale and salt, stir to combine and cover with a lid, cook for 10 minutes over medium heat, then take the lid off and cook for 30 more minutes.
A few minutes before the lentils are cooked stir in mushroom and truffle sauce and grate some parmesan cheese over it, stir to combine. The lentils should be tender but still hold their shape and the kale should be soft.