Preheat the oven to 350ºF.
Render the bacon over medium-high heat until crispy. Drain on paper towels and reserve.
While the bacon is rendering, bring the cream to a simmer, whisk in the Romano, white cheddar cheese, and nutmeg, and bring to a simmer.
Combine the cornstarch with 3 tablespoons of water and whisk into the cheese sauce to thicken.
Remove from the heat and reserve.
Grease a 9x9 baking dish.
Layer the 1/3 of the potatoes topped with 1/3 of the onions, 1/4 of the bacon, and 1/3 of the cheese sauce.
Repeat two more times, then cover with foil and bake for about an hour, or until the potatoes are just tender.
Remove the foil, top with the sharp cheddar and bake for another 30 minutes, or until the cheese is just beginning to brown and the potatoes are fork tender.
Remove the potatoes from the oven and top with remaining bacon and scallions. Serve immediately.