Peel the Russet potatoes and dice them into uniform 1/2-inch cubes. Dice the onion and mince the garlic. Shred both cheddar and Monterey Jack cheeses (if using) and set aside.

In a large Dutch oven or stock pot, melt 2 tablespoons of unsalted butter over medium heat. Add the diced onion and cook, stirring occasionally, until softened and translucent, about 5-7 minutes.

Add the minced garlic and cook for another minute until fragrant, being careful not to burn it.

Pour in the chicken broth and water. Add the diced potatoes, salt, and black pepper. Bring the mixture to a boil, then reduce the heat to medium-low, cover, and simmer for 15-20 minutes, or until the potatoes are very tender and easily pierced with a fork.

While the potatoes are simmering, prepare the roux in a separate small saucepan. Melt 4 tablespoons of unsalted butter over medium heat. Once melted, whisk in the all-purpose flour and cook for 1-2 minutes, stirring constantly, to create a smooth paste (roux).

Slowly whisk in the whole milk to the roux, a little at a time, until smooth and no lumps remain. Continue to cook and whisk until the mixture thickens, about 3-5 minutes. Stir in the heavy cream and remove from heat.

Once the potatoes are tender, lightly mash about half of the potatoes directly in the pot with a potato masher or the back of a spoon. This will help thicken the soup while still leaving some chunks for texture. (Alternatively, for a smoother soup, you can blend about 1-2 cups of the soup mixture in a blender and return it to the pot, or use an immersion blender.)

Pour the thickened milk mixture (roux) into the potato soup, stirring constantly until well combined and heated through. Do not boil after adding the milk mixture.

Reduce the heat to low. Gradually add the shredded cheddar cheese and Monterey Jack cheese (if using) to the soup, stirring constantly until the cheese is completely melted and the soup is smooth and creamy. Taste and adjust seasonings (salt and pepper) as needed.

Ladle the creamy cheesy potato soup into bowls. Garnish generously with crumbled bacon, fresh minced chives, and a dollop of sour cream if desired. Serve hot and enjoy!