In a bowl, whisk hot water, bouillon, Worcestershire, ketchup, garlic powder, cornstarch, and pepper. Set aside.
In another bowl, mix beef, breadcrumbs, dry mustard, ketchup, Worcestershire, bouillon, onion powder, salt, and pepper. Shape into 4 patties.
Heat olive oil in a skillet; brown patties on both sides. Remove and reserve 1 tbsp drippings.
Sauté onions 3–4 minutes, then add garlic and butter. Cook 2 minutes.
Deglaze with ½ cup gravy mixture, scraping browned bits. Add mushrooms; cook 2–3 minutes. Stir in remaining gravy base. Simmer 5 minutes.
Return patties to pan, coat with gravy, and simmer 3–5 minutes until heated through. Serve hot.