Heat the olive oil and 1 tbsp butter in a large skillet over medium-high heat. Add the mushrooms in a single layer and sear undisturbed for 3 to 4 minutes until deep golden brown on the underside.

Push mushrooms to the edges, add the remaining 2 tbsp butter to the center, then cook the sliced onion for 3 minutes over medium heat until soft. Add the minced garlic and stir for 30 seconds until fragrant.

Pour in the beef broth and Worcestershire sauce, scraping up any browned bits. Add the salt, pepper, and thyme. Simmer over medium heat for 5 to 7 minutes, stirring occasionally, until the liquid reduces by half and coats the mushrooms in a dark glaze.

Spoon into a serving bowl, scatter the chopped parsley over the top, and serve immediately.