1. Preheat the oven to 350°F (180°C). Butter a 13 x 9″ (3 L) glass baking dish.
2. In a large pot, melt butter over medium heat; add onion and sauté for 8 minutes, or until starting to brown. Add vinegar and boil, stirring and scraping up brown bits, until evaporated. Whisk flour into milk; gradually pour into pot, whisking constantly. Cook, stirring, for about 5 minutes or until bubbling and thickened. Remove from heat; stir in parsley, rosemary, and salt and pepper. Add potatoes; toss gently to coat in sauce.
3. Spread evenly into the prepared dish. Sprinkle with cheese. Bake, uncovered, for about 45 minutes or until golden and bubbling and potatoes are tender. Let stand for 10 minutes before serving.
Recipe and photo are provided by Dairy Farmers of Canada. For more recipe ideas, visit dairygoodness.ca.