Preheat oven to 400 degrees F and grease a 9 x 13-inch baking dish with cooking spray. Set aside.
Melt butter in a large sauté pan over medium heat.
Add onion and cook until softened, about 5-10 minutes.
Add garlic and cook an additional 30 seconds.
Sprinkle on the flour and whisk for 1 minute.
While whisking, slowly pour in the broth.
Stir in the heavy cream, 1 teaspoon salt, pepper, paprika, cayenne and nutmeg.
Bring to a simmer and simmer for 5 minutes, stirring regularly (don’t let it boil).
Remove from the heat and set aside.
Place sliced potatoes into a large bowl and toss with remaining 1 teaspoon salt (this will draw out some liquid, just discard).
Spread half of the potatoes in an even layer on the bottom of the baking dish.
Top evenly with half of the cream sauce.
Sprinkle evenly with half of the cheddar cheese and half the parmesan.
Top evenly with the remaining sliced potatoes, the remaining cream sauce, and the remaining cheddar cheese and parmesan.
Cover the pan with aluminum foil that's been sprayed with cooking spray and bake for 30 minutes.
Remove the foil and bake uncovered for 20-30 minutes, or until the potatoes are cooked through.
Cool for 20 minutes, blot any grease, then slice and serve warm!
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