Start the mushrooms: Set a large deep skillet with a lid on the stove over medium-high heat.

Add the olive oil and butter to the skillet.

When the butter melts, add the sliced mushrooms to the skillet in an even layer.

Cook the mushrooms for 4 minutes without stirring, until they look darker and browned on the bottom.

Stir the mushrooms and cook for 3 to 4 minutes more, until they have shrunk and browned in spots.

Add the chopped onion and 1/2 teaspoon of the kosher salt to the skillet.

Cook the onion for 4 minutes, stirring often, until it softens and turns lightly golden.

Add the sliced garlic to the skillet.

Cook for 30 seconds, stirring, until the garlic smells fragrant.

Simmer the barley: Add the rinsed pearl barley to the skillet.

Stir the barley for 1 minute so it gets coated in the fat and mixed with the mushrooms and onion.

Pour the vegetable broth into the skillet. Scrape the bottom of the pan with a wooden spoon to loosen any browned bits, then bring the broth to a boil over high heat.