How to Make It
In a small saucepan, cover the lentils with 1 inch of water and add a large pinch of salt. Cover and bring to a boil. Simmer over low heat, stirring occasionally, until tender, about 20 minutes. Drain.
In a large saucepan, cook the pancetta over moderately low heat, stirring, until most of the fat has been rendered, about 8 minutes. Add the garlic, carrots, celery, onion, potato, celery root, squash, rutabaga and bay leaf and cook over moderately high heat for 2 minutes, stirring to coat with fat. Season with salt and pepper and cook until the vegetables soften, about 3 minutes longer. Add the chicken stock and bring to a boil. Reduce the heat to low and simmer until the vegetables are tender, about 20 minutes. Discard the bay leaf.
Stir in the lentils, parsley and thyme and season with salt and pepper. Ladle the soup into bowls, top with Gruyère shavings and serve piping hot.