To the rendered bacon fat, add the onion and garlic and cook until the onion softens, about 5 minutes. Whisk in the flour and cook for 30 seconds then whisk in the chicken stock and milk. Season with 2 teaspoons of salt and a good amount of pepper. Add potatoes and bring the mixture to a boil and reduce to a simmer. Cook until potatoes are tender, about 12-15 minutes.