Instructions:

Season beef with salt and pepper and toss in flour to coat.
In a large skillet over medium-high heat, or in the insert of your instant pot if you’re using that, melt butter. Add beef in batches and sear on all sides until brown and crusty. Remove to a plate and continue searing beef until complete.
Add onions and cook briefly, just 2 minutes or so, scraping up bits from bottom of pan with a wooden spoon.
TO MAKE IN THE SLOW COOKER:

Transfer beef and onions to slow cooker and add mushrooms, garlic powder, onion powder, dried thyme, rosemary, bouillon cube, Worcestershire sauce, bay leaf, pepperoncinis, and 2 cups beef stock. Cover and cook on high for 3-4 hours or low for 6-7 hours.
30 minutes before cooking time is over, whisk cornstarch into 3 tablespoons water and add to slow cooker. Cover and continue cooking for remaining 30 minutes. Season to taste and enjoy!