Bring a large pot of salted water to a boil. Cook the noodles according to the package directions, drain, and toss with 1 tablespoon of the butter. Cover to keep warm.

Meanwhile, heat 1 tablespoon of the olive oil in a large, deep skillet over medium-high heat. Add half the mushrooms in a single layer and cook without stirring for 3 minutes, then stir and cook 2 to 3 minutes more, until browned. Transfer to a bowl and repeat with the remaining oil and mushrooms. Cooking in two batches lets the mushrooms brown instead of steam.

Lower the heat to medium. Melt the remaining 2 tablespoons butter in the skillet, add the onion and cook for about 5 minutes, until soft and golden. Stir in the garlic, thyme and smoked paprika and cook for 30 seconds, until fragrant.

Sprinkle the flour over the onions and stir for 1 minute. Slowly pour in the broth, stirring and scraping up the browned bits from the bottom of the pan, then stir in the soy sauce and mustard. Simmer for 3 to 4 minutes, until the sauce thickens.

Return the mushrooms and any juices to the skillet, season with the salt and pepper, and simmer for 2 minutes.

Take the skillet off the heat. Stir a few spoonfuls of the hot sauce into the sour cream to warm it, then stir the sour cream into the skillet until smooth and creamy. Do not let the sauce boil after this point, or the sour cream may curdle. Taste and add more salt if needed.

Spoon the stroganoff over the buttered noodles and sprinkle with parsley. Serve right away.