Heat the olive oil in a large pot over medium heat. Add the onion and cook until it softens.
Stir in the garlic, parsnips, and celery root. Cook for 3 to 4 minutes until the vegetables start to brown slightly.
Add the lentils, apple cider, vegetable broth, thyme, and bay leaf. Stir to combine.
Bring the mixture to a boil, then lower the heat to a simmer. Cover and cook for 25 to 30 minutes until the lentils are tender.
Remove the bay leaf. Season with salt and pepper to taste.
Ladle into bowls and top with the chopped parsley before serving.