Heat oven to 350°F.
Toss sweet potatoes and parsnips with flour, salt, and pepper in a large bowl.
Heat butter, heavy cream, and sage in a medium pot over medium heat until butter is completely melted.
Remove from heat and let sit for 10 minutes.
Add a layer of vegetables to the bottom of a lightly greased 9 x 13-inch baking dish, slightly overlapping.
Add 11⁄2 ounces of Gorgonzola and about 1⁄4 cup of cream mixture on top of the vegetables.
Repeat layering process until entire mixture is used.
Cover the pan with foil and bake for 1 hour.
Remove foil and bake another 20 to 30 minutes, or until the potatoes and parsnips are soft and a golden-brown crust forms.
Serve warm or at room temperature.