Mix beef, panko, egg, onion, Worcestershire, mustard, garlic powder, salt, and pepper; form 4 patties.
Cook patties in oil for 4-5 minutes per side; remove.
Melt butter, cook mushrooms, and onion for 5-6 minutes; add garlic, cook 1 minute.
Stir in flour for 1 minute; whisk in broth, Worcestershire, mustard, and pepper; cook 3-4 minutes until thick.
Return patties, simmer for 2-3 minutes.
Garnish with parsley; serve with potatoes and tea.