In a bowl, combine beef, chopped onion, beaten egg, breadcrumbs, Dijon mustard, Worcestershire sauce, garlic powder, salt and pepper. Mix until just combined, then divide into 8 portions and shape into patties.

Heat oil in a large frying pan over medium-high. Cook half the patties for 4 minutes without moving until golden underneath. Flip and cook a further 4 minutes until golden and cooked through. Transfer to a plate and cover; repeat with remaining patties.

Melt butter in the same pan. Add sliced onion and cook, stirring often, for 5 minutes until softened and golden.

Add mushrooms and cook for 3 minutes until they start to brown.

Stir in garlic and cook for 30 seconds until fragrant. Add tomato paste and flour, cooking for 1 minute.

Pour in beef stock and water, stirring constantly until the gravy thickens. Add Worcestershire sauce and season with salt and pepper.

Return the rissoles to the pan. Reduce heat to low, cover and simmer for 10 minutes until cooked through.

If using, stir through cream and heat for 1 minute until warmed.