**Prepare the Steak**
Season the steak cubes with salt, black pepper, garlic powder, and onion powder. In a large skillet, heat the olive oil over medium-high heat. Sear the steak bites in batches until browned on all sides, about 2-3 minutes per batch. Transfer the seared steak to the crockpot.
**Add Vegetables (Optional)**
If using, add the sliced onions, bell peppers, and mushrooms to the crockpot, distributing them evenly over the steak bites.
**Prepare the Sauce**
In a mixing bowl, whisk together the beef broth, soy sauce, Worcestershire sauce, brown sugar, cornstarch, minced garlic, dried thyme, and red pepper flakes until well combined. Pour the sauce mixture over the steak and vegetables in the crockpot.
**Cook the Steak Bites**
Cover and cook on low for 4-6 hours or on high for 2-3 hours, until the steak is tender and the sauce has thickened.
**Thicken the Sauce (If Needed)**
If you prefer a thicker sauce, remove a ladleful of the sauce from the crockpot and whisk it with a tablespoon of cornstarch until smooth. Return the mixture to the crockpot and stir well. Cook on high for an additional 15-30 minutes, until the sauce thickens to your liking.
**Serve**
Garnish with freshly chopped parsley and serve the steak bites over rice, pasta, mashed potatoes, or enjoy them as an appetizer with toothpicks.