In a large pot, heat olive oil over medium heat. Add the mushrooms and sauté until the liquid has released and evaporated, about 5-6 minutes.
Add the diced onion and sliced carrots, cooking 3-5 minutes until the onions are translucent.
Stir in the thyme and flour, mixing well to coat the vegetables. Cook for about 1-2 minutes.
Add the brown lentils and vegetable broth. Stir to combine. Bring the mixture to a boil, then add the milk. Reduce the heat to low and simmer uncovered for 20 minutes, or until the lentils are tender, stirring occassionally.
Remove from the heat and stir in the sour cream, salt (to taste), and black pepper. Serve warm.
Video
Notes
Lentils: Rinse the dried lentils under cold water and pick out any stones or debris before cooking. You do not need to soak them before cooking.Storage: To store leftovers, let the soup cool completely and transfer it to an airtight container. Refrigerate for up to 4 days. Freezing is not recommended because the dairy can cause the soup to change texture.