Stir the panko and milk together in a large mixing bowl and let sit for 5 minutes while you grate the onion. Add the onion, egg, salt, allspice, nutmeg and pepper and mix. Add the turkey and mix with your hands for about 30 seconds, just until no dry streaks remain.

Wet your hands and roll the mixture into about 20 meatballs, 45 g (3 tbsp) each and roughly 4 cm / 1.5 inches wide. Set them on a plate.

Heat 1/2 tbsp of the olive oil in a 30 cm / 12-inch deep skillet over medium-high heat until it shimmers. Brown 10 meatballs for 6 to 7 minutes, turning every couple of minutes, until browned on all sides, then move them to a clean plate. Repeat with the remaining oil and meatballs. Leave the browned bits in the pan.

Lower the heat to medium and melt the butter in the same skillet. Whisk in the flour and cook for 1 minute until it smells toasty. Slowly whisk in the broth, scraping up the browned bits, then add the Worcestershire and mustard. Simmer for 3 minutes until the gravy coats the back of a spoon.

Return all the meatballs and any juices to the skillet. Partly cover and simmer on low for 10 minutes, turning once, until the largest meatball reads 165 F / 74 C. While they simmer, boil the noodles in 3 L of water with the salt in a 5-quart / 4.7 L pot for 7 to 8 minutes until al dente, then drain.

Take the skillet off the heat and wait 1 minute until the bubbling stops. Whisk the sour cream with a ladle of hot gravy in a small bowl, then stir it into the pan. Don't let the gravy boil again. Taste and add salt if needed, then serve the meatballs and gravy over the noodles with parsley and a spoonful of jam.