In a large saucepan over medium heat, melt the butter. Add the finely chopped onion and sauté until translucent, about 5 minutes. Add the minced garlic and cook for an additional 1-2 minutes until fragrant.

Pour in the chicken or vegetable broth, stirring gently to combine with the sautéed onions and garlic. Increase the heat to medium-high and bring the mixture to a gentle boil.

Once the broth is boiling, reduce the heat to low and slowly whisk in the instant mashed potato flakes. Continue to stir until the potato flakes are fully dissolved, resulting in a smooth consistency.

Stir in the milk, mixing thoroughly to achieve a creamy texture. Season the soup with salt and freshly ground black pepper to taste. Allow the soup to simmer on low heat for an additional 5 minutes, stirring occasionally.

Remove the saucepan from heat. Ladle the hot potato soup into bowls and garnish with your choice of toppings, such as shredded cheddar cheese, chopped chives, or crumbled bacon.