PREHEAT oven to 400°F; MEASURE the GROUP ONE ingredients into a small bowl (3 tablespoons parmesan cheese, 1 1/2 teaspoons kosher salt and 1/2 teaspoon ground black pepper); MIX well with fork; SET aside.
PEEL and slice potatoes thinly; PEEL and slice onions thinly (into rings); GRATE 3 cups sharp cheddar cheese.
LIGHTLY grease a 2.5 quart dish or 9x13 baking dish; LAYER 1/3 of the potatoes, 1/2 of the onion, 1/3 of the cheese, and 1/3 of the GROUP ONE mixture.
REPEAT layers; TOP with remaining potatoes, the remaining GROUP 1 mixture, and the remaining cheese.
POUR 1 cup heavy cream into a one-quart measuring cup; POUR 3/4 cup strong chicken broth; POUR 1/4 cup dry white wine to the 2-cup mark; ADD 1 teaspoon rubbed sage, 1/2 teaspoon prepared horseradish (or 1/2 teaspoon dry mustard), 2 pinches white pepper and 2 pinches ground nutmeg; WHISK well; POUR mixture evenly over potatoes.
SPRINKLE 1/8 cup parmesan cheese, 2 pinches paprika, 1 dash freshly cracked pepper and 1/4 teaspoon chives or parsley, evenly over the top of mixture in baking dish for the TOPPING.
BAKE 1 hour.
ALLOW to stand at least 7 minutes to cool and set, before serving.
GET READY to melt into your seat after just one taste!