Root gratin with anchovy cream and breadcrumbs

Name: Root gratin with anchovy cream and breadcrumbs
Prep time: 15 min
Cook time: 1 h 30 min
Total time: 1 h 45 min
Cuisine: European, British, French, Mediterranean
Ingredients: 100g unsalted butter; 2 large onions sliced; 4 garlic cloves sliced; 30g anchovy fillets roughly chopped; 2 tbsp thyme; 250ml double cream; 300ml chicken stock; 1 swede sliced thin; 1 celeriac sliced thin; 400g potatoes sliced thin; 150g white breadcrumbs; 1 tbsp thyme; Few sprigs rosemary chopped; 60g cheddar gruyere mixed; Olive oil; Salt and pepper; Pepper