Drain the the clams over a small bowl, saving the juice.
Pour clam juice into a 2 cup measuring cup and add water/fish stock to the clam juice to make a total of 2 cups of liquid. Set aside.
Press Sauté on the Instant Pot and add the chopped bacon. Cook, stirring occasionally, until fat has rendered out of it, but not yet crispy.
Add the butter, onion, celery, and thyme. Cook until the onions begin to turn translucent. During the cooking process scrape the bottom of the pot to deglaze and loosen any brown bits.
Add the garlic, salt, and pepper. Cook for 1 minute, stirring frequently.
Add the potatoes and clam juice mixture and stir. Push Cancel.
Lock the lid and place valve to the sealing position.
Pressure Cook/Manual on HIGH pressure for 4 minutes.
When cook time has ended, allow for 4 minutes Natural Pressure Release, then do a controlled Quick Release by slowly releasing the valve to the venting position.
Carefully open the pot and stir.
Use a potato masher to carefully mash a few of the potatoes to thicken the liquid.
Turn the Sauté function on LESS and add the clams and half and half.
If desired thicken by removing ½ cup of the hot liquid and whisking in the corn starch or arrowroot powder. Whisk well and stir the mixture back into pot until desired consistency is achieved.
Serve warm and garnish with chopped green onions.