For the Slow Cooker:

1. Season the beef tips with salt and pepper, then dredge in flour.
2. In a skillet, melt butter over medium-high heat. Sear the beef until browned on all sides and transfer to the slow cooker.
3. In the same skillet, add onions, garlic, and mushrooms. Sauté for 3-4 minutes until softened, then add to the slow cooker.
4. Pour in beef broth, red wine (if using), Worcestershire sauce, thyme, and rosemary. Stir to combine.
5. Cover and cook on low for 6-8 hours or high for 3-4 hours until beef is tender.
6. About 15 minutes before serving, add the cornstarch slurry, stir, and cover to thicken the sauce.

For the Instant Pot:

1. Season and flour the beef tips as above.
2. Set the Instant Pot to sauté, add butter, and sear the beef until browned. Remove and set aside.
3. Add onions, garlic, and mushrooms, sauté briefly, then add the beef back in.
4. Pour in beef broth, red wine (if using), Worcestershire sauce, thyme, and rosemary, stirring well.
5. Seal the lid and set to High Pressure for 20 minutes. Natural release for 10 minutes, then quick release any remaining pressure.
6. Set back to sauté, add the cornstarch slurry, and cook until thickened.