In a large stockpot over medium heat saute onion, celery, and garlic in 1.5 tbsp olive oil for 3-4 minutes. Add fresh thyme and chopped sage and continue to saute for 2 more minutes. Add a generous pinch of salt and black pepper stir well.
Add chopped root vegetables to pot (carrot, parsnip, potato). Add stock, stir well. Bring to a boil, reduce heat to a low simmer and cook for 20-25 or until vegetables are tender.
In the meantime, Saute mushrooms in a skillet with 1/2 tbsp olive oil and fresh thyme. You can skip this step and just add mushrooms to the pot. Sauteeing the mushrooms in a pan separately ensures a perfectly cooked mushroom, browned and crisp on the outside. Do not crowd the mushrooms in a pan or they will steam, in this case you may as well toss them in the pot with the other veggies. Once mushrooms are browned and crisp season with salt and pepper.
Add mushrooms to the stew, or alternatively you can add browned mushrooms to each individual bowl of stew. Taste for seasoning, add more salt and pepper as needed.
Make the sour cream slurry; in a small bowl whisk together sour cream and a ladle of stew broth. Stir sour cream slurry into the stew. Stir well.
Top with chopped parsley, serve hot.
NOTES: TO MAKE A THICKER STEW:Add several ladles of stew into a blender and blend to thicken, stir the pureed stew into the stew. I have made the stew both ways and personally preferred it without the extra thickening, however, if you prefer a super thick stew go for it.