Halve the potatoes and cut sunchokes into 1-inch pieces and place in a large baking dish and season with salt and pepper.
Meanwhile, put the cream, butter, sage, and garlic in a small saucepan and bring to a simmer over medium heat. Cook for about 5 minutes, to infuse the cream with flavor. Season with salt and pepper.
Using a fine mesh strainer, pour the cream mixture over the potatoes, and disard the garlic and sage. Cover the top with the grated Parmesan and bake for 30 to 35 minutes, until the potatoes and sunchokes are cooked through and top begins to brown. Cover and keep warm until ready to serve.