Peel and dice the russet potatoes and finely chop the small onion. Combine chopped potatoes and onions in a large stock pot or Dutch oven and cover with cold water.

Place the pot over medium-high heat and bring the water to a boil. Once boiling, reduce heat slightly and let it simmer for about 15-20 minutes until the potatoes are fork-tender.

Drain excess water, leaving about a cup of liquid to help create a creamy texture.

Mash the cooked potatoes and onions directly in the pot, aiming for a chunky consistency.

Stir in the butter and milk thoroughly until well combined. Optionally use an immersion blender for a smoother finish.

Return the pot to medium heat and let the soup cook for an additional 6-7 minutes, stirring frequently.

Season the soup with salt and pepper to taste, then stir well to ensure every spoonful is flavorful.