In a large bowl, grate the onion and mix it with panko breadcrumbs to soak up the juices. Add beef mince, grated zucchini, grated carrot, crushed garlic, and egg. Season with Worcestershire sauce, salt, pepper, and herbs if desired.

Shape the mixture into 12 equal patties, pressing an indent into one side of each to prevent doming.

Heat a skillet over medium-high heat and add olive oil. Place the rissoles in the pan, sear for 1.5 minutes on each side until golden brown.

In the same skillet, reduce heat to medium, add butter, and melt. Add sliced mushrooms and sauté until golden brown, about 5 minutes. Then add grated onion and sauté for an additional minute. Stir in crushed garlic until aromatic.

Sprinkle flour over the mushroom mixture, stirring for 1-2 minutes to create a roux. Gradually pour in beef stock while whisking to avoid lumps. Allow to come to a gentle simmer.

Nestle the browned rissoles into the gravy, cover, and simmer for 4-5 minutes.

Serve over buttery mashed potatoes or your favorite starch, ladle plenty of gravy over the top.