In a large pot over medium-high heat, crisp the diced thick-cut bacon for about 5–7 minutes until golden and crunchy. Remove the bacon with a slotted spoon and place it on a paper towel-lined plate to drain, reserving about 2 tablespoons of the drippings in the pot.

Using the reserved bacon fat, add the diced onions to the pot and sauté for 7–10 minutes over medium heat until they turn soft and golden brown. Add minced garlic in the last minute of cooking.

Stir in the dried basil, kosher salt, black pepper, and celery salt, letting them cook together for about 1 minute.

Pour in the chicken broth while scraping the bottom of the pot to deglaze it. If using a slow cooker, transfer the sautéed mixture into it at this point.

Add the diced russet potatoes to the mixture and stir to combine. Cover the pot tightly, reducing the heat to low, and simmer for 15-25 minutes until the potatoes are tender.

Once the potatoes are tender, blend them in the pot using an immersion blender to your desired consistency. Stir in the heavy cream, butter, sour cream, and adjust the seasonings as needed.

Ladle the soup into bowls and top with crispy bacon bits, shredded cheddar cheese, diced chives, or a dollop of sour cream.