Preheat oven to 400°F (200°C). Pierce potatoes with a fork, rub with oil, and bake for 60 minutes or until tender. Once cooled, peel and cut into 1/2-inch cubes.
In a large Dutch oven over medium heat, cook diced bacon until crispy. Use a slotted spoon to remove bacon bits and set aside for garnish, reserving approximately 2 tablespoons of rendered fat in the pot.
Add the butter to the bacon fat. Once melted, add the diced onion and sauté until translucent and soft, approximately 5-7 minutes.
Whisk in the flour to create a roux. Cook for 2 minutes, stirring constantly to remove the raw flour taste without browning.
Gradually whisk in the chicken broth, ensuring no lumps form. Bring the mixture to a gentle simmer.
Slowly pour in the heavy cream while whisking. Stir in the sugar, salt, black pepper, and celery salt.
Add the cubed baked potatoes to the pot. Simmer on low heat for 10-15 minutes, allowing the starches to naturally thicken the soup.
Taste and adjust seasoning. For a thicker consistency, lightly mash a few potato chunks against the side of the pot.
Ladle into bowls and top generously with shredded cheddar cheese, reserved bacon bits, and sliced green onions.