Creamy Potato Gratin with sautéed onion, garlic, and rosemary | British Foodie

Ingredients

• 500 g potatoes (preferably Maris Piper or King Edward, thinly sliced)
• 1 medium onion (finely chopped)
• 2 cloves garlic (minced)
• 1 tsp fresh rosemary (chopped (or ½ tsp dried))
• 150 ml double cream
• 150 ml milk
• 100 g crème fraîche
• 100 g grated cheese (such as Gruyère or Cheddar)
• 1 pinch salt and black pepper (to taste)
• 1 knob butter (for greasing the dish)