Heat 1 tablespoon of the vegetable oil in a large skillet over medium heat until shimmering. Add the mushrooms and 1/4 teaspoon salt, cover the skillet and cook until the mushrooms are softened, about 5 minutes. Uncover and continue to cook until the mushrooms are dry and browned, 5 to 10 additional minutes transfer the mushrooms to a 6-quart slow cooker lined with a Reynolds Kitchens® Slow Cooker Liner.
Heat the remaining 2 tablespoons vegetable oil in the same skillet over medium-high heat until shimmering. Add the onions, tomato paste, garlic, porcini and thyme, and cook until the onions are softened and lightly browned, 8 to 10 minutes. Stir in the flour and cook for 1 minute. Slowly whisk in the broth, scraping up any browned bits and smoothing out any lumps. Transfer the mixture to the slow cooker.
Stir the wine, soy sauce and bay leaves into the slow cooker. Season the beef with salt and pepper and place in the slow cooker, nestling it in the liquid. Cover and cook on low until the beef is tender, 9 to 11 hours (or on high for 5 to 7 hours).
Once finished cooking, let the stew settle for 5 minutes, then remove fat from the surface using a large spoon and discard the bay leaves. In a small bowl, stir together 1 cup of the hot cooking liquid with the sour cream and Dijon, then stir the sour cream mixture into the slow cooker. Stir in the dill, season with salt and pepper, and serve.