Cut the cod into 1-inch pieces (2.54 cm) and cut the shrimp and oysters in half. Chop the onion and potatoes into ½-inch dice (1.27 cm). Dice the carrot into ¼-½ inch pieces (0.6-1.27 cm) and chop the celery into ¼-inch pieces (0.6 cm). Mince the garlic cloves.
Set the Instant Pot to high sauté. Heat the oil in the pot and sauté the onion, carrot, and celery for 5-10 minutes. You want the veggies to soften, not brown.
Add the potatoes, garlic, and flour to the pot. Stir for 30 seconds.
Pour in the clam juice and chicken broth, and sprinkle in the Old Bay and dried thyme. Put the lid on the pot and close the valve.
Pressure cook or 5 minutes using the “beans/chili” setting under high pressure. Allow the pressure to release naturally for 10 minutes before carefully opening the vent on the lid.
Add the cod, chopped shrimp, drained chopped oysters, corn, and cream to the pot. Simmer the chowder on high sauté for 5-10 minutes, stirring frequently to prevent sticking.
The fish should reach a safe internal temp of 145°F (63°C). You can check a few pieces with a meat thermometer if you’d like to be extra cautious.
Ladle the chowder into soup bowls and garnish (if desired) with fresh herbs and black pepper. Enjoy!