Dic-'tator casserole

Sausage, spicy Italian, cut into tiny ½-inch rounds	1 pound
Sausage, bratwurst, cut into tiny ½-inch rounds	1 pound
Oil, extra-virgin olive	3 tablespoons +
Garlic, thinly sliced	5 cloves
Bell pepper, seeded, cored, sliced ½-inch thick	1 red + 1 green
Onion, ½-inch slice	1 large
Tomato sauce	¼ cup
Water	2 tablespoons
Parsley, chopped	3 tablespoons
Salt	1 ½ teaspoons +
Pepper, white, freshly ground	½ teaspoon +
Potatoes, Yukon Gold, peeled, quartered	1 ½ pounds
Cream, heavy	1 cup
Milk, whole	½ cup
Butter, unsalted, cut into chunks	6 tablespoons
Horseradish, bottled, drained	¼ cup
Parmesan, grated	½ cup

1. Preheat the oven to 400°F. 
2. Place the sausage on a rimmed baking sheet, and liberally drizzle with oil. 
3. Roast until the casings are crisp and the juices run clear, about 15 to 20 minutes; let rest, and switch the oven to the broiler setting. 
4. While the sausages roast, heat 3-tablespoons oil in a skillet over medium heat, add the garlic, and cook for 2 to 3 minutes. 
5. Add the peppers and onion, increase the heat, and cook until the vegetables are tender, about 5 to 6 minutes. 
6. Add the tomato sauce, water, salt and pepper to taste, and reduce heat to a simmer. 
7. In a large pot, cover the potatoes with salted cold water, then simmer, uncovered, until very tender, about 25 minutes. 
8. In a small saucepan, bring the cream, milk, butter, 1 ½-teaspoon salt, and ½-teaspoon white pepper just to a simmer, stirring until the butter melts; keep warm and covered. 
9. Drain the potatoes, dump into a large bowl, and mash until smooth. 
10. Stir in the warm cream mixture, followed by the horseradish, and then the cheese. 
11. Toss the sausages, peppers, and onion with the tomato mixture and parsley; transfer to a casserole. 
12. Top the tiny dicks with mashed ‘tators, spreading evenly, and then broil until golden brown.