For the croutons: Preheat the oven broiler on low. Toss the bread with oil, garlic powder, salt, and pepper. Place on a foil lined baking sheet. Toast under the broiler until crunchy and golden brown on all sides. Then remove and set aside.
Melt the butter with the oil in a pot on medium heat. Once hot, sauté the celery, garlic, and onion until soft.
Add the chicken broth, potatoes, and scallions. Season well with salt and pepper, bring to a simmer partially covered with a lid. Simmer until the potatoes are soft.
In a blender, pulse until the soup thickens. Blend until smooth or somewhat chunky depending on your preference. Season with salt and pepper to taste. (Add broth to thin out the soup if too thick)
Top with croutons and scallions.
Notes
Adjust the portions as needed to batch cook for use later in the week.