Preheat oven to 400 degrees. Wash and peel parsnips. Slice parsnips as thin as you can - a mandolin slicer works great here. See below for my favorite handheld version, which makes slicing super simple! Slice onion at the same width.

In a 9 by 9 baking dish that's been sprayed with cooking spray, thinly arrange parsnips and onion. Layer with gruyere cheese, reserving a 1/2 cup for the topping.

In a small saucepan, combine half and half, whole milk, salt, garlic, sage leaves, and nutmeg over medium heat to a simmer. Heat for 5 minutes. Remove from heat and allow to steep for an additional 5 (longer will result in a stronger sage flavor). Pour mixture evenly over parsnips and onions. Top with remaining gruyere and parmesan cheese.

Top with foil, and bake for 60 minutes. Remove foil and cook an additional 10 minutes.

Check to confirm parsnips are cooked through - depending on how thick or thin your slices are, you may need to cook longer. Serve warm!