In a large pot, heat the butter and olive oil over medium heat.
Add the mushrooms and cook until tender and golden, 4-5 minutes. Remove to a plate (keeping as much liquid/oil in the pot as possible).
Add the leeks and carrots, and cook, stirring occasionally, until starting to soften, 5-6 minutes.
Add the broth, dill, salt, pepper and bay leaf.
Stir in potatoes; the liquid will just barely cover the potatoes, most likely. If it doesn't, add enough extra broth to just cover the potatoes. Bring the mixture to a simmer, cover, and cook until the potatoes are tender, 15-20 minutes (depending on how large or small you chopped them).
Optional step: for a slightly thicker soup, right now, take out the bay leaf, grab a potato masher, and lightly mash the potatoes right in the pot until desired consistency before proceeding.
Stir the mushrooms back into the soup.
Whisk or blend tog ether the milk, sour cream and flour (I prefer to use a blender for an ultra-lump free mixture).
Quickly stir the milk mixture into the soup, and heat until slightly thickened, 2-3 minutes. Season to taste with additional salt and pepper, if needed.
Serve with crumbled bacon or fresh dill, if desired.
Mushrooms: I prefer portabella or baby bella mushrooms for a richer flavor, but white button mushrooms can be used, too. Leeks: also, as noted in the post above, you could probably sub in another type of chopped onion for the leek, but I highly encourage the use of leeks if at all possible (because, yum). See the post for a quick tutorial on trimming, cutting and washing leeks.