In a large mixing bowl, combine grated onion, panko breadcrumbs, beef mince, grated zucchini, grated carrot, crushed garlic, egg, and optional herbs. Season with Worcestershire sauce, salt, and black pepper to taste. Mix everything until well incorporated, ensuring a smooth mixture.

Once your mixture is well combined, divide it into 12 equal portions. Gently shape each portion into a patty about 1 inch thick, making a small indent in the center to prevent doming during cooking.

Heat a large non-stick pan over medium heat and add a drizzle of cooking oil. Once hot, carefully place the rissoles in the pan without overcrowding, searing them for 1.5 minutes on each side until they are golden brown.

In the same pan, add unsalted butter and allow it to melt over medium heat. Once melted, sauté diced onions and chopped mushrooms until they are tender and the onions are translucent, about 5 minutes.

Gradually pour in half of the beef stock, stirring to ensure there are no lumps. Add the previously cooked rissoles back to the pan, along with the remaining stock, Worcestershire sauce, salt, and black pepper to taste. Bring the mixture to a gentle simmer, covering the pan.

Once the rissoles are cooked and the gravy has reached a creamy consistency, plate the rissoles generously topped with the mushroom gravy.