Add diced onion to the pot and cook until golden brown, stirring occasionally, about 10 minutes.
Stir in flour until the onions are well coated.
Gradually pour in milk, stirring constantly for 1 minute.
Incorporate the mashed potatoes, breaking them up and stirring until smooth, about 10-15 minutes on medium-low heat. Season with salt and pepper to taste.
Serve the soup in bowls, garnished with dried parsley, shredded cheddar cheese, diced turkey bacon, and a sprinkle of cracked black pepper. DEVOUR!