In a large mixing bowl, grate half an onion into panko breadcrumbs. Add beef mince, grated zucchini, and carrot, along with one egg, minced garlic, and Worcestershire sauce. Mix gently until combined.

Divide the mixture into 12 equal portions. Shape each portion into round patties about 1 inch thick.

Heat 2 tablespoons of olive oil in a large skillet over medium-high heat. Add the rissoles, cooking for about 1.5 minutes on each side until golden-brown. Remove from skillet.

In the same skillet, melt 1 tablespoon of butter over medium heat. Add a finely chopped onion, and sauté until softened. Stir in sliced mushrooms and cook until browned.

Sprinkle in 2 tablespoons of flour, stirring for about 1 minute. Gradually pour in beef stock, stirring to prevent lumps. Simmer until thickened, about 3-5 minutes.

Return the rissoles to the skillet, cover, and let them simmer in the gravy for 4-5 minutes.

Remove from heat and serve the smothered rissoles over mashed potatoes or preferred side, garnishing with parsley.