Bring salted water to a boil in a large pot and add the pasta. Cook according to package instructions until al dente. Reserve 120 ml of pasta water, then drain the pasta.

In a large skillet, heat the vegan butter over medium-high heat. Add the sliced onion and sauté for 5 minutes until translucent.

Add the sliced mushrooms to the skillet and cook for 8 to 10 minutes, stirring occasionally, until they are browned and most of the moisture has evaporated.

Stir in the thyme, chopped garlic, and smoked paprika, cooking for an additional 30 seconds until fragrant.

Sprinkle the flour over the mushroom mixture and stir for 1 minute to cook the flour slightly.

Gradually whisk in the vegetable broth, ensuring there are no lumps. Add the soy sauce and Dijon mustard, then let the mixture simmer for 4 to 6 minutes until it thickens.

Reduce the heat and stir in the unsweetened plant milk and vegan sour cream until the sauce is smooth and creamy. Season with salt and black pepper to taste.

Add the cooked pasta to the sauce, mixing well. If the sauce is too thick, add a little reserved pasta water to reach the desired consistency. Heat for 1 to 2 minutes until warmed through.

Serve hot, garnished with fresh parsley, a sprinkle of black pepper, or a squeeze of lemon for brightness. Optionally, add spinach or peas for color.