Spray a skillet with cooking spray, and add the sausage. Cook over medium-high heat until browned, about 5 minutes, turning links often.
Reduce heat to medium-low. Carefully add ½ cup water to skillet. Cover and simmer for 12 minutes or until cooked through.
Place the potatoes in a large saucepan with 2 teaspoons of salt and enough water to cover the potatoes.
Bring to a simmer and cook for 15 to 20 minutes or until the potatoes are very tender.
Drain the potatoes in a colander and let them cool for a few minutes. Press potatoes through a potato ricer or use a potato masher.
Return potatoes to the pan. Add the butter, sour cream, milk, mustard, and season with salt and pepper. Whisk over a low heat.
Taste for seasoning—mashed potatoes love salt and pepper.
Place a mound of potatoes on each plate and top with two sausages. Serve.