Set your Instant Pot to sauté mode and heat the olive oil for about one minute until shimmering.
Add the chopped bacon and cook it, stirring occasionally, until it's nice and crispy and the fat has melted out, which should take around eight minutes.
Use a slotted spoon to transfer the bacon pieces to a paper towel-lined plate to drain.
Add the chopped onion and sliced celery to the bacon fat in the pot and sauté for about two minutes until they begin to soften.
Stir in the minced garlic and ground thyme and cook everything together for another minute to release their aromas.
Turn off the sauté function, then add the diced red potatoes and chicken broth to the pot.
Seal the lid and lock it in place, then cook on high pressure for eight minutes once the pot reaches full pressure; it may take around ten minutes to build pressure.
Once cooking is complete, allow the pressure to release naturally over about ten minutes before opening the lid.
In a small bowl, whisk together two tablespoons of milk and the cornstarch until fully combined into a slurry.
Stir this slurry and the remaining cup of milk into the soup inside the pot.
Season to your taste with salt and black pepper, then let the soup sit for about ten minutes, giving it a few stirs as it thickens.
Ladle the soup into bowls and sprinkle the reserved crispy bacon pieces on top before serving.
