Pat the beef cubes dry with paper towels. In a large bowl, toss them with the salt, pepper, and flour until evenly coated.

Heat the olive oil in a large Dutch oven or heavy-bottomed pot over medium-high heat. Working in batches, sear the beef cubes on all sides until deeply browned, about 2-3 minutes per side. Remove to a plate and set aside.

In the same pot, add the diced onion and cook for 3-4 minutes until softened. Add the garlic and mushrooms, cooking for another 4-5 minutes until the mushrooms release their moisture and begin to brown.

Pour in the beef broth, using a wooden spoon to scrape up all the browned bits from the bottom of the pot. Stir in the Worcestershire sauce, tomato paste, dried thyme, and bay leaves.

Return the seared beef and any accumulated juices back to the pot. Bring to a simmer, then reduce the heat to low. Cover and cook for 1.5 to 2 hours, until the beef is fork-tender.

When the beef is tender, prepare the cornstarch slurry. In a small bowl, whisk the water and cornstarch until smooth. Slowly stir the slurry into the simmering gravy, continue to cook for 3-5 minutes, stirring constantly, until thickened.

Discard the bay leaves, taste for seasoning, and adjust with more salt and pepper if needed. Garnish with fresh parsley and serve hot over mashed potatoes, rice, or egg noodles.