Grate the onion finely and mix it with panko breadcrumbs in a large bowl to absorb moisture. Add the ground beef, grated zucchini, carrot, garlic, egg, Worcestershire sauce, and dried herbs, if using. Combine everything gently until just mixed. Form the mixture into 12 patties, each about 1/4 cup in size, creating a slight indentation on one side for even cooking.

In a large non-stick pan, heat a splash of oil over medium-high heat until shimmering. Cook the rissoles in batches, for approximately 1.5 minutes on each side, until they appear golden brown but not fully cooked. Remove them from the pan and set them aside on a plate, allowing them to rest while you prepare the gravy.

Wipe the pan clean with a paper towel and return it to the heat. Melt the unsalted butter over medium heat until bubbly. Add the sliced onion and sauté until slightly softened, about 3 minutes. Stir in the sliced mushrooms, cooking until they are browned and tender, and then add the garlic, cooking for an additional minute until fragrant.

Sprinkle the flour over the onion and mushroom mixture, stirring continuously for about one minute. Gradually whisk in the beef stock, ensuring there are no lumps until the gravy becomes smooth and thick. Allow the gravy to simmer for a few moments so it can develop depth while you prepare to return the rissoles.

Carefully place the browned rissoles back into the pan, making sure they're coated in the rich mushroom gravy. Cover the pan and let them simmer gently for 4-5 minutes, or until the gravy thickens and the rissoles are cooked through. You’ll know they’re ready when they reach an internal temperature of 160°F and are no longer pink inside.

To serve, plate the rissoles and generously ladle the luscious mushroom gravy on top. This comforting dish pairs beautifully with creamy mashed potatoes or fluffy rice. Garnish with fresh parsley for a touch of color and enjoy a delightful meal that brings warmth to your dining table!