Use a paper towel to wipe off any dirt from your mushrooms. Slice into ¼-inch sized pieces.
Heat 2 tablespoons olive oil in a large skillet on medium-high heat. Once the oil is shimmering, add HALF of the prepped mushrooms without overcrowding them. To get a nice sear, do not touch the mushrooms for 4-5 minutes. Quickly flip the mushrooms, and sear the other side for 4-5 minutes, being sure not to stir/move them around. You want to get them nicely browned.
Reduce the heat to medium-low and add HALF of the garlic, HALF of the thyme, and ¼ teaspoon salt. Cook for 1-2 minutes, then transfer to a plate or bowl.
Repeat this process with the remaining oil, mushrooms, garlic, thyme, and salt. After flipping mushrooms, add diced onion and cook for 5-7 minutes, until translucent.
While the mushrooms cook, prepare the vegetable broth roux. In a medium bowl, whisk together the vegetable broth, soy sauce, and flour until smooth and free of clumps.
Add first batch of cooked mushrooms back into the pan. Pour in the white wine to deglaze the pan, scraping up any brown bits on the bottom of the pan. Simmer for about 3 minutes, until most of the liquid has evaporated.
Add the vegetable broth roux to the pan, whisking to remove any lumps. Bring to a simmer, then stir in nutritional yeast, paprika, and black pepper. Cook over medium-low heat for 8-10 minutes, until thick and creamy.
While the sauce cooks, make the pasta. Bring a large pot of salted water to a boil and cook the pasta according to the package instructions until al dente. Drain and set aside, keeping warm.
Remove skillet with mushroom mixture from the heat, and stir in mustard and sour cream. Add the cooked pasta and toss to coat. Serve immediately.