Wild mushroom & chestnut cottage pie

Name: Wild mushroom & chestnut cottage pie
Prep time: 20 min
Cook time: 1 h
Total time: 1 h 20 min
Cuisine: British
Ingredients: 2 tbsp vegetable oil; 2 carrots chopped; ½ swede chopped; 12 pearl onions peeled and left whole; 1 garlic clove crushed; 1 rosemary sprig; 1 tsp tomato purée; 1 tsp yeast extract, such as Marmite; 200g can chopped tomatoes; 50ml white wine; 175ml vegetable stock; 500g fresh penny buns/ceps or mixed wild mushrooms roughly chopped; 200g vacumn-packed chestnuts halved; 3 potatoes diced; 2 parsnips diced; 2 carrots diced; 300g celeriac diced; 100g butter; 50ml milk