Season beef with salt and pepper, toss to coat. Heat olive oil in a large non-stick pan or skillet to medium-high heat.
Add half of the beef and cook 3 to 4 minutes, rotating to brown each side. Remove pieces from pan and place in a slow cooker. Repeat for remaining beef.
Add onions to the same pan without cleaning it. Reduce heat to medium and cook 5 minutes, stirring often, until fragrant and slightly translucent. Add garlic, stir and cook 1 minute more. Transfer onion mixture to the slow cooker.
Add wine to the pan and scrape the bottom to loosen up leftover browned bits. Cook 1 to 2 minutes to reduce slightly.
Stir together beef stock, corn starch, and Worcheshire sauce. Add to pan and bring to a low boil, stirring constantly. Simmer 1 to 2 minutes until thickened slightly. Stir in oregano.
Pour sauce over meat and onions in the slow cooker. Top with mushrooms and add bay leaves.
Cover and cook on HIGH for 4 hours or LOW for 8 hours. Remove bay leaves, stir sour cream into the sauce, and serve over the desired amount of cooked noodles or rice.
Browning the meat ahead of time creates a nice sear and adds flavor to the sauce.
Don't discard the browned bits or clean the pan after cooking the meat and veggies. Dry white wine is used to deglaze the pan and incorporate into the sauce.
When adding corn starch, be sure to stir constantly until thickened to prevent clumping.
Add sour cream at the very end just before serving to prevent curdling while the meat cooks.