Sauté the Aromatics: Set the Instant Pot to Sauté mode on High or More. Melt the butter, then add the diced onion and chopped celery. Stir occasionally and cook for 3–5 minutes until softened. Stir in the flour, ensuring all vegetables are well coated.
Add the Liquids and Potatoes: Pour in the broth, scraping the bottom of the pot to loosen any bits of flour. Add the cooking sherry, all reserved clam juice (but not the clams yet), bay leaves, celery leafy tops, and diced potatoes. Stir well.
Pressure Cook: Secure the lid and set the valve to Sealing. Select Manual/Pressure Cook on High for 5 minutes. Once finished, perform a quick release and carefully remove the lid.
Final Simmer and Seasoning: Switch the Instant Pot back to Sauté mode on High. Stir in the drained clams, heavy cream, seasoned salt, black pepper, and Old Bay. For extra spice, add the Zatarain’s concentrate. Let simmer until the chowder starts to bubble, then hit Keep Warm/Cancel.
Thickening Option: For a richer texture, mix 2 tablespoons cornstarch with 2 tablespoons water and stir it in after adding the cream and clams. Let it simmer for 2 minutes until slightly thickened.