Mustard Gratin with Potatoes & Parsnips from Home Made Christmas by Yvette van Boven

Ingredients

• A handful of breadcrumbs
• Sea salt
• ½ cup plus 2 tablespoons (75 g) all-purpose flour
• 5 tablespoons (75 g) butter
• 1⅔ pounds (750 g) parsnips, cut into ½-inch (1.5-cm) cubes
• 1⅔ pounds (750 g) waxy potatoes, cut into ½-inch (1.5-cm) cubes
• 2 teaspoons fresh thyme leaves
• 2 tablespoons white wine vinegar
• 2 tablespoons coarse mustard
• About 2½ cups (600 ml) milk
• freshly ground black pepper
• About ¼ cup (25 g) grated Parmesan cheese