Creamy Potato Gratin With Onion, Garlic And Rosemary

Name: Creamy Potato Gratin With Onion, Garlic And Rosemary
Prep time: 15 min
Cook time: 1 h 15 min
Total time: 1 h 30 min
Cuisine: British
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)