Season the beef on all sides with thyme, salt, and pepper. Heat 1 tablespoon olive oil in a heavy pan on medium-high heat (or use your slow-cooker on sear) and sear the beef well, turning until deep brown on all sides. Remove the beef, and set aside.
Step 2
Melt 2 tablespoons butter in the hot pan. Add the mushrooms (do not crowd; this can be done in batches), season generously with salt and pepper, and toss to coat. Allow to cook undisturbed for at least 3-4 minutes, then turn and cook on the other side.
Step 3
Add 2 tablespoons butter to the pan with the mushrooms. Once melted, add the onion and garlic. Cook, stirring occasionally, until softened and fragrant, 5-6 minutes. Add the remaining 2 tablespoons butter, allow to melt, and add the flour, cooking until browned. Stir in the mustard, Worcestershire sauce, and soy sauce.
Step 4
Pour in the red wine while whisking, and cook down for 3-4 minutes. Then add the stock. Return the beef to the pan, or add the liquid and beef to your slow-cooker. Cover and cook on your stovetop for 2 ½ hours, or in your slow-cooker (8 hours on low or 4 hours on high) until beef is tender.
Step 5
Once the beef has cooked, test the sauce—you want it to be thick enough to coat the back of a spoon. If it is not, stir the cornstarch into the sour cream, and set aside in a medium heatproof bowl.
Step 6
Remove 1-2 ladlefuls of liquid, add to a small measuring cup, and allow to cool for one minute. Whisk the liquid into the bowl of sour cream and cornstarch. Repeat twice to temper the sour cream (this will ensure a smooth sauce), then add the sour cream to the beef and stir to combine. If using a slow-cooker, increase to high (or sear) and cook for 5-10 more minutes or until thickened, or simmer on your stovetop.
Step 7
Prepare the pasta while the stroganoff finishes cooking. Serve the stroganoff over the pasta, and top with fresh parsley or dill.