In a large bowl, combine ground meat, crushed saltine crackers (or panko), buttermilk (or egg), parsley, thyme, garlic powder, onion powder, salt, and pepper until just combined. Shape into 16 1-1/2-in meatballs.
Heat the Instant Pot in sauté mode. When hot, add 2 tablespoons of oil and brown the meatballs in batches, about 2 minutes per side. Transfer browned meatballs to a plate.
Add the remaining tablespoon of oil to the pot, add sliced mushroom and garlic and cook for 2-3 minutes before stirring; cook until fragrant, about 2 more minutes.
Add the broth to the pot, deglaze the pot using a wooden utensil with a flat head to scrape all of the stuck bits of chops off of the bottom of the pot. Press “cancel”
Return meatballs to the pot, add heavy cream and bay leaf; mix to combine. Secure lid and close pressure-release valve. Select “Pressure cook” adjust the time to 10 minutes. When the time is over, let the pressure release on its own for 5 minutes. Then, carefully turn the valve to “Venting,” to release any extra pressure that might still be in there. Press “cancel”. Then remove the lid.
Remove bay leave and discard.
Mix in a small bowl the cold water and cornstarch until no lumps.
Select “sauté” mode and adjust to less. Add cornstarch slurry, sour cream, and mustard; mix to combine. Cook for 2-3 minutes until the sauce thickens.
Sprinkle with fresh parsley, if desired. Serve immediately over mashed potatoes, rice or noodles.