In a large bowl, combine ground beef, onion, parsley, breadcrumbs, and egg. Season with salt and pepper, then mix until well combined.
Shape the mixture into small patties using your hands.
Heat 1 tablespoon of olive oil in a large skillet over medium heat. Cook the patties for about 4-5 minutes on each side until browned and cooked through. Remove from the skillet and set aside.
In the same skillet, add another tablespoon of olive oil and cook the mushrooms until they are brown and tender.
Sprinkle the flour over the mushrooms and stir to coat the mushrooms evenly.
Gradually pour in the beef stock, stirring continuously, until the gravy thickens.
Return the rissoles to the skillet, smothering them with the mushroom gravy, and simmer for an additional 5 minutes.
Serve hot, with a side of mashed potatoes or steamed vegetables if desired.