Properly clean the leeks: Slice off the dark green tops and root ends. Halve the remaining white and light green parts lengthwise, then chop them. Submerge the chopped leeks in a large bowl of cold water and swish them vigorously to release any trapped grit. Let them sit for a minute, then lift the clean leeks out of the water, leaving the dirt behind.

Set your Instant Pot to the 'Sauté' function on normal heat. Once hot, add the butter. When melted, add the cleaned leeks and a generous pinch of salt. Cook, stirring occasionally, for 5-7 minutes until the leeks are very soft and fragrant.

Add the minced garlic and cook for one more minute until aromatic, being careful not to let it burn.

Pour about a half-cup of the vegetable broth into the pot to deglaze. Use a wooden spoon to scrape up any flavorful browned bits from the bottom. Press 'Cancel' to turn off the Sauté function.

Add the cubed potatoes, fresh thyme sprigs, bay leaf, and the remaining vegetable broth to the pot. Stir to combine.

Secure the lid, ensure the steam release valve is set to 'Sealing,' and pressure cook on high for 8 minutes.

Once the cooking cycle is complete, let the pressure release naturally for 10 minutes. Then, perform a quick release to vent any remaining steam.

Carefully open the lid. Remove and discard the thyme sprigs and the bay leaf.

Using an immersion blender, blend the soup directly in the pot until it is completely smooth and velvety. Alternatively, carefully transfer the soup in batches to a regular blender and return it to the pot.

With the Instant Pot on the 'Keep Warm' setting or turned off, slowly stir in the heavy cream until incorporated. Taste the soup and season generously with salt and freshly ground black pepper until the flavors are to your liking. Serve hot.