4 ounces full-fat cream cheese
1 medium yellow onion
2 medium stalks celery
3 cloves garlic
2 to 2 1/2 pounds potatoes
4 slices bacon (about 4 ounces)
1 1/2 teaspoons kosher salt
1/4 teaspoon freshly ground black pepper
1 (32-ounce) box low-sodium chicken broth (4 cups)
1 cup whole milk
2 tablespoons cornstarch
4 ounces sharp cheddar cheese, shredded (about 1 cup)
2 medium scallions
Cut 4 ounces cream cheese into 16 pieces and let it soften at room temperature while you prepare the soup. Prepare the following, placing them all in the same medium bowl: Dice 1 medium onion and 2 medium celery stalks, and mince 3 garlic cloves. Peel 2 to 2 1/2 pounds russet potatoes, cut into 1/2-inch cubes, and place in a separate bowl.
Tear 4 slices bacon in half crosswise and place in a 6-quart or larger electric pressure cooker or Instant Pot in a single layer. Cook with the Sauté function until the bacon fat is rendered and the bacon is browned, flipping halfway through, 6 to 8 minutes total. Transfer the bacon to a paper towel-lined plate. Drain all but 2 tablespoons of the rendered bacon fat.
Add the onion, celery, garlic,1 1/2 teaspoons kosher salt, and 1/4 teaspoon black pepper to the pot. Cook on the Sauté function, stirring occasionally, until softened, about 5 minutes. Turn the pressure cooker off. Add the potatoes and 1 box chicken broth.
Lock on the lid and make sure the pressure valve is set to seal. Set to cook for 10 minutes under HIGH pressure. It should take about 15 minutes to come up to pressure. Meanwhile, place 1 cup whole milk and 2 tablespoons cornstarch in a small bowl and whisk until the cornstarch is dissolved. Shred 4 ounces sharp cheddar cheese (about 1 cup) on the large holes of a box grater, thinly slice 2 medium scallions, and crumble the bacon.
When the cook time is up, let the pressure naturally release for 10 minutes. Quick release any remaining pressure.
Turn on the Sauté function. When the soup is simmering, whisk the milk again to dissolve the cornstarch. Stir the milk and cream cheese cubes into the soup, and simmer until the soup is thickened and the cream cheese is melted, about 4 minutes.
Turn off the cooker and let the soup cool 10 minutes before serving so that it can thicken. Divide the soup between bowls and top with the crumbled bacon, cheddar cheese, and scallions.