Lightly grease a 13x9-inch baking pan and set it aside.
Put the potato cubes in a large saucepan with enough water to cover them. Bring the contents to a boil and cook the potatoes for about 15 minutes until they are tender.
Drain the water and set the potatoes aside.
In a large saucepan, melt 1 tablespoon of butter over medium heat.
Add the sausage, onion, and bell pepper, and cook for about 10 minutes until the onion is tender.
Transfer the sausage mixture to a plate and set it aside.
Add the potatoes to a large bowl and mash or beat them until smooth.
Mix in the remaining butter, sour cream, mayonnaise, milk, Cajun spice, salt, and pepper until well combined.
Gently fold in the sausage mixture and 1 ½ cups of cheddar cheese.
Transfer the potato mixture to the prepared baking dish.
Sprinkle the remaining ½ cup of cheddar cheese over the top of the casserole.
Bake in the preheated oven for 25 to 30 minutes until hot and bubbly.