Begin by heating 15 ml of olive oil in a large, wide frying pan over medium heat. Add the finely diced onion and sauté gently until soft, translucent, and fragrant. Once cooked, transfer the onion to a plate and allow it to cool completely.

Return the pan to medium-high heat, drizzle in a little more olive oil, and add the sliced portabellini mushrooms. Sauté the mushrooms until their moisture has evaporated and they begin to brown slightly. Season them with a light sprinkle of salt, a grind of black pepper, and the dried oregano. Remove from the heat and set aside.

In a mixing bowl, combine the lean beef mince with the cooled onion mixture, breadcrumbs, egg, chopped parsley, mixed spice, nutmeg, salt, and freshly ground pepper. Mix thoroughly using your hands until the ingredients are well incorporated and the mixture holds together. Shape the mixture into 22 to 25 meatballs of equal size for consistent cooking.

Heat another 15 ml of olive oil in the same pan and brown the meatballs in batches, turning them until all sides develop a deep, golden crust. Transfer the cooked meatballs to a plate with the mushrooms and set aside.

To make the sauce, add the butter to the pan drippings and heat until melted and bubbling. Stir in the flour and cook for about 2 minutes, forming a smooth roux. Gradually whisk in the hot chicken stock, stirring constantly to prevent clumps. Add the Worcestershire sauce, Dijon mustard, and lemon juice. Pour in the fresh cream and stir to combine, then bring the mixture to a gentle boil.

Carefully return the browned meatballs and sautéed mushrooms to the sauce, turning to coat them fully. Let the mixture simmer for 4 to 5 minutes, allowing the meatballs to cook through and the sauce to thicken to a luscious consistency.

Finish with a scattering of fresh parsley and, if desired, a light drizzle of lemon-infused olive oil for added brightness. Serve piping hot with a side of buttered tagliatelle or creamy mashed potatoes for a comforting and satisfying meal.