Begin by cutting the beef sirloin steak into 1-inch cubes. Optionally, heat 2 tablespoons of olive oil in a skillet over medium-high heat, then sear the steak cubes for 1-2 minutes on each side until golden brown.

In the same skillet, add roughly chopped onions and sauté until soft and translucent, about 3-4 minutes. Stir in minced garlic for an additional minute until fragrant.

In a mixing bowl, combine beef broth, Worcestershire sauce, soy sauce, Dijon mustard, dried thyme, and black pepper. Whisk until thoroughly combined.

Layer the seared steak cubes in the crockpot, followed by the sautéed onions and garlic mixture. Pour the prepared sauce over the meat, ensuring all pieces are well-coated.

Cover the crockpot and cook on low for 6-8 hours or high for 3-4 hours, checking for doneness. The steak should be fork-tender and juicy when served.

For a thicker sauce, mix cornstarch with cold water to create a slurry. About 30 minutes before serving, stir this mixture into the crockpot.

Let the crockpot steak bites rest for 10-15 minutes with the lid on before serving.