Heat the cooking oil in a large pot over medium heat. Add the diced onion and garlic, and saute for 2-3 minutes, or until softened and fragrant.
Add the carrots and celery root, then stir to combine.
Stir in the rinsed lentils, bay leaves, tomato paste, vinegar, vegetable seasoning (or crumbled stock cube), salt, pepper, and water. Bring to a boil.
Reduce the heat to low, cover, and let it simmer for 35-40 minutes, or until the lentils are tender. Stir occasionally to prevent sticking.
To thicken your lentils, prepare the roux by melting the butter in a small pan over medium heat. Whisk in the flour and cook for 2-3 minutes until golden brown.
Stir in the roux to the lentils and cook for a further 5 minutes to thicken the sauce. If too thick, add some beef broth or water, to your liking.
Serve with Späetzle and Seitenwurst, or as preferred.