Set the pressure cooker to low saute (or use a skillet on low). Once it's hot, melt the butter then add the leeks. Saute the leeks on low for 7-8 minutes.
Add the garlic. Saute 1 minute.
Add the thyme, bay leaf, potatoes, and stock. Cover the cooker and set it for high pressure, 7 minutes.
When the time is up, let the cooker natural release for 15 minutes, then press the quick release and finish venting the pressure.
Open the lid and remove the bay leaf. Then use a stick blender to blend the soup until it is smooth. (Or transfer the soup to a blender in two batches.)
Stir in the half and half. Then taste and add salt and pepper as needed.
Serve soup with toppings such as croutons, minced chives, crumbled bacon, or a swirl of cream.