- Preheat oven to 350°F. Mist a 9 x 9-inch baking dish with cooking spray and set aside.
- In a small bowl, combine lemon zest, panko, Parmesan and rosemary. Set aside.
- In a large saucepan on medium, melt 1 tbsp butter. Add onions and cook, stirring occasionally, until very soft and pale in color, 3 to 5 minutes. Reduce heat to medium-low and continue cooking, stirring occasionally, until onions begin to brown and start sticking to bottom of pan, 5 to 7 minutes more. Add garlic and cook until fragrant, about 30 seconds. Add vinegar; stir to coat.
- Push onions to one side and melt remaining 1 tbsp butter on other side. Add flour and cook, stirring frequently, until flour is golden, 1 to 2 minutes. Add broth and stir everything together, scraping up browned bits from bottom of pan. Add cream and milk and bring to a simmer until slightly thickened, about 30 seconds. Remove pan from heat; set aside.
- Peel potatoes and slice 1/8 inch thick with a knife or a mandoline. Place 2 tbsp sauce on bottom of prepared dish. Arrange half of potato slices in dish, overlapping slightly, and sprinkle with ¼ tsp each salt and pepper; cover with half of remaining sauce. Arrange remaining potatoes; sprinkle with remaining 1/4 tsp each salt and pepper and cover with remaining sauce. Cover dish with foil; place on a baking sheet. Bake until potatoes are tender, 45 minutes to 1 hour. Remove foil and sprinkle with panko mixture. Bake until panko is golden, about 15 minutes.
- Serving Size 1/10 of gratin
- Calories 204
- Carbohydrate Content 29 g
- Cholesterol Content 23 mg
- Fat Content 8 g
- Fiber Content 3 g
- Protein Content 4.5 g
- Saturated Fat Content 5 g
- Sodium Content 158 mg
- Sugar Content 3 g
- Monounsaturated Fat Content 0 g
- Polyunsaturated Fat Content 0.5 g