Fry the bacon in a 5-liter (about 1.3 gallon) pot using a little oil. Add the thyme. Once it gets a bit of color, add the onion. When the onion becomes translucent, add the carrots, parsnip, and celery. Add enough water to almost fill the pot. Season with salt and pepper and let to boil.
When the veggies are almost done, add the potatoes.
When the potatoes are done, remove from the heat.
Put the sour cream in a bowl and gradually add and whisk some hot soup until the mix reaches the soup temperature. Pour everything into the large pot and continue mixing.