Heat the olive oil in a large pot over medium heat. Add the onion and cook until softened.
Stir in the carrots, parsnips, and potatoes. Cook for 5 minutes, stirring occasionally.
Add the mushrooms and garlic. Cook until the mushrooms release their liquid and begin to brown.
Pour in the lentils, vegetable broth, and thyme. Bring to a boil, then reduce to a simmer.
Cover and cook for 25 minutes, or until the lentils and vegetables are tender.
Remove from heat and stir in the heavy cream. Season with salt and pepper.
Ladle into bowls and top with fresh parsley before serving.