Peel each potato, then cut 1/3 off of one and set aside. Cut remaining 2 1/2 potatoes into medium chunks and add to a bowl.
Peel onion, cut into medium chunks, and set aside.
Set Instant Pot to "Sauté" and add 1 tablespoon cooking fat. Once melted and hot, add onion and stir to combine. Cook onion, stirring every so often, until softened and starting to brown, about 5 minutes. Mince garlic and add to the pan. Stir constantly until fragrant, about 1 minute.
Press “Cancel” on the pot, then add all potatoes, broth, and sea salt to the pot. Fit the lid on the pot, lock into place, then select the “Soup” setting (or "Manual," "High pressure"), and set the timer for 10 minutes.
While the pot comes to pressure, cook your bacon and prepare the “Cheesy” Croutons, if using.
Once the soup finishes cooking, move the lever on the lid to “Vent” to manually release the pressure. Be careful with the steam!
Use a hand blender (or potato masher for a chunkier soup) to blend until soup is smooth. Add nutritional yeast, lemon, and coconut milk, and blend again until combined. Adjust seasonings to taste and add more liquid for a thinner soup, or until it reaches your desired consistency.
Add toppings if desired and serve warm.