Season the beef with kosher salt and freshly ground black pepper, then toss with flour until lightly coated.

In a large skillet over medium-high heat, or in the Instant Pot insert if using the Instant Pot method, melt the butter.

Add the beef in batches and sear on all sides until browned and crusty. Remove the browned beef to a plate and repeat until all beef is seared.

Add the diced onion to the pan and cook for about 2 minutes, scraping up the browned bits from the bottom with a wooden spoon.

Slow Cooker Method

Transfer the beef and onions to the slow cooker.

Add the sliced cremini mushrooms, garlic powder, onion powder, dried thyme, dried rosemary, beef bouillon cube, Worcestershire sauce, bay leaf, sliced pepperoncinis, and 2 cups beef stock.

Cover and cook on high for 3 to 4 hours or on low for 6 to 7 hours, until the beef is tender.

About 30 minutes before the cooking time is finished, whisk the cornstarch with 3 tablespoons of water until smooth.

Stir the cornstarch slurry into the slow cooker.

Cover and continue cooking for the remaining 30 minutes, or until the gravy thickens.

Remove the bay leaf, season to taste with salt and pepper, and serve warm.

Instant Pot Method

Add the seared beef back to the Instant Pot with the onions.

Add the sliced cremini mushrooms, garlic powder, onion powder, dried thyme, dried rosemary, beef bouillon cube, Worcestershire sauce, bay leaf, sliced pepperoncinis, and 1 1/2 cups beef stock.

Secure the lid and select the “Meat/Stew” setting, or cook on high pressure for 35 minutes.

Once the cooking time is complete, let the pressure release naturally for 10 minutes.

Carefully switch the pressure release valve to “venting” to release any remaining pressure.

Turn on the “Sauté” setting.

In a small bowl, stir the cornstarch with 3 tablespoons of water until smooth.

Stir the cornstarch slurry into the cooking liquid.

Let the gravy simmer until thickened.

Remove the bay leaf, season to taste with salt and pepper, and serve warm.