Combine ground turkey, egg, bread crumbs, onion, garlic, salt, pepper, allspice, and nutmeg in a large bowl; mix until well combined. Use a spring-hinged scoop to form about 12 meatballs.

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Turn on a multi-functional pressure cooker (such as Instant Pot®) and select Saute function. Add butter and oil; heat until butter is melted. Cook meatballs in the hot fat until browned on all sides, about 4 minutes. Work in batches if necessary. Transfer browned meatballs to a shallow dish and loosely cover with foil.

Pour broth into the pot and scrape off all the browned bits from the bottom of the pot using a wooden spoon. Mix in 1/2 cup half-and-half, Dijon mustard, and Worcestershire sauce. Return meatballs to the pot and coat with the sauce.

Cancel Saute mode. Close and lock the lid. Select high pressure according to manufacturer’s instructions; set timer for 5 minutes. Allow 10 to 15 minutes for pressure to build.

Release pressure using the natural-release method according to manufacturer’s instructions, for 7 minutes. Release remaining pressure carefully using the quick-release method, about 5 minutes. Unlock and remove the lid. Transfer meatballs carefully to a shallow dish and cover loosely with foil.

Whisk together remaining 1/4 cup half-and-half, sour cream, and flour in a small bowl. Stir into the pot and simmer until thickened, 3 to 5 minutes. Serve meatballs with sauce and garnish with parsley.