Ingredients:

  • 2 lbs (900g) Yukon Gold or Russet potatoes, peeled
  • 2 tbsp (30 ml) olive oil
  • 1 tsp salt
  • 1/2 tsp black pepper
  • 1 large yellow onion, thinly sliced
  • 1/2 cup (60g) all-purpose flour
  • 1/4 tsp salt
  • 1/4 tsp black pepper
  • Vegetable oil, for frying (approx. 2 cups / 475 ml)
  • 3 tbsp (45g) unsalted butter
  • 3 tbsp (25g) all-purpose flour
  • 3 cups (710 ml) whole milk
  • 1/2 tsp salt
  • 1/4 tsp black pepper
  • 1/4 tsp nutmeg (freshly grated)
  • 1 cup (115g) shredded Gruyere cheese
  • 1 cup (115g) shredded sharp cheddar cheese

Instructions:

  1. Prepare the Crispy Onions: Toss sliced onions with flour, salt, and pepper. Fry in hot vegetable oil until golden brown and crispy. Drain on paper towels. Set aside.
  2. Prepare the Potatoes: Peel and thinly slice the potatoes (about 1/8 inch thick). Toss with olive oil, salt, and pepper.
  3. Make the Cheese Sauce: Melt butter in a saucepan. Whisk in flour and cook for 1 minute to create a roux. Gradually whisk in milk until smooth. Bring to a simmer, then reduce heat and cook, stirring constantly, until thickened (about 5-7 minutes). Stir in salt, pepper, and nutmeg. Remove from heat and stir in Gruyere and cheddar cheeses until melted and smooth.
  4. Assemble the Gratin: Layer half of the potato slices in the prepared baking dish, slightly overlapping. Pour half of the cheese sauce over the potatoes. Repeat with the remaining potatoes and cheese sauce.
  5. Bake: Bake in a preheated oven at 375°F (190°C) for 45-55 minutes, or until the potatoes are tender and the top is golden brown and bubbly.
  6. Top and Rest: Sprinkle the crispy fried onions evenly over the top of the gratin. Let stand for 10-15 minutes before serving.