Scorzonera Casserole with Mashed Potatoes and Herbed Crème Fraiche

I make this easy casserole all year round, using scorzonera in winter, Belgian endive in the fall, and asparagus in spring. Scorzonera is also known as black salsify or black oyster plant, because its flavor is reminiscent of oyster. It can sometimes be difficult to find in markets, so feel free to substitute it with asparagus or endive. I peel the roots under water, so they keep their creamy white color.