Adjust oven rack to middle rack position and heat oven to 450 degrees. Melt 1 tablespoon butter in Dutch oven over medium-high heat. Cook onion until softened, about 30 seconds. stir in 2 cups cream, 1 cup broth, Yukon Golds, nutmeg, 2 teaspoon salt, and 1 teaspoon pepper.
Bring to boil, then reduce heat to medium and simmer until potatoes are translucent at edges and mixture is slightly thickened, about 10 minutes. Off heat stir in lemon zest and juice.
Transfer potato mixture to 13 by 9 inch baking dish and bake until bubbling around edges and surface is just golden, about 20 minutes. Meanwhile, melt remaining 2 tablespoons butter in 12-inch nonstick skillet over medium-high heat.
Cook shredded potatoes until beginning to brown, about 2 minutes. Add remaining 1/2 cup cream, remaining 1/2 cup broth, and 1/2 teaspoon pepper to skillet and cook, stirring occasionally, until liquid has evaporated, about 3 minutes. off heat, stir in 1/2 cup parmesan and 2 tablespoons chives.
Remove baking dish from oven and top with shredded potato mixture. sprinkle with remaining Parmesan and continue to bake until top is golden brown. Let cool for 15 minutes then sprinkle with remaining chives. Enjoy.