Turn your instant pot to sauté mode. Once your pot becomes hot, add in your oil. then add in your white onions. Stir and cook for about three minutes

Add to your instant pot the onions, clams, and cornstarch. Mix well

Add in your potatoes, salt, pepper, water, and half-and-half. Stir well

Place your lid on the instant pot. Set your instant pot to high pressure for 3 minutes

Once your instant pot is done cooking, do a quick release by bringing the valve toward you and opening the pressure

Open the instant pot once it has come to a full release

Choose to cancel on your instant pot and then select the sauté mode. Stir your soup occasionally for about 10 minutes to reduce the liquid, and continue to cook and thicken your half-and-half

Add more salt and pepper to your liking. Pour your soup into a bowl or a bread bowl, top with parsley (optional), and enjoy!