Prepare the meatball mixture: In a large bowl, combine the minced beef, minced turkey, grated onion, breadcrumbs, beaten egg, allspice, nutmeg, salt, and pepper. Use your hands to mix gently until everything is just incorporated. You will feel the mixture become slightly sticky and cohesive, but avoid overworking it or the meatballs will turn tough.

Shape the meatballs: Roll the mixture into uniform balls, roughly 2.5cm in diameter. You should get about 30 to 35 meatballs. As you roll them, you will notice they feel firm but moist to the touch. Place them on a baking tray lined with baking paper.

Brown the meatballs: Melt the butter in a large frying pan over medium-high heat. Working in two batches to avoid overcrowding, add the meatballs and cook for 4-5 minutes, turning occasionally. You are looking for a deep golden-brown crust on all sides, and you will hear a satisfying sizzle as they hit the pan. The smell of browned butter and spices will fill your kitchen.

Create the gravy base: Once the meatballs are browned, remove them from the pan and set them aside. Do not wipe the pan clean. Sprinkle the flour into the remaining butter and pan juices. Whisk continuously for 1-2 minutes until the mixture turns a light tan colour and smells nutty — this is your roux.

Deglaze and combine: Slowly pour in the beef stock while whisking constantly. You will see the mixture thicken and become smooth as it comes to a gentle simmer. Stir in the double cream and Worcestershire sauce. The gravy will be pale beige and velvety, with a rich, savoury aroma.

Slow cook everything: Place the browned meatballs into the slow cooker. Pour the creamy gravy over the top, ensuring all the meatballs are covered. Set the slow cooker to low and cook for 4 hours. The meatballs will become incredibly tender, and the gravy will deepen in colour to a warm caramel tone.

Finish and serve: Just before serving, taste the gravy and adjust the seasoning with a pinch more salt or pepper if needed. Garnish with fresh parsley. The meatballs should be fork-tender, and the gravy will be luxuriously thick and smooth.