Celeriac vegan shepherd's pie

Name: Celeriac vegan shepherd's pie
Prep time: 30 min
Cook time: 2 h
Total time: 2 h 30 min
Cuisine: British
Ingredients: 2 tbsp / 30 ml olive oil; 2 large banana shallots OR 1 onion, finely diced; 5 large garlic cloves, finely diced; 250 g / 8¾ oz chestnut mushrooms, finely diced; 2 sprigs rosemary, leaves chopped finely; 8 sprigs thyme, leaves picked; 1 mushroom stock cube (or homemade porcini stock*); 1 tbsp red miso; 1 large carrot, finely diced; 2 celery stalks, finely diced; 1 heaped tsp smoked paprika; a good pinch of chilli (optional); salt, adjust to taste; black pepper, to taste; 120 ml / ½ cup vegan red wine (or stock); 1 x 400 g / 7 oz tins quality peeled plum tomatoes; 2 tbsp tomato paste / concentrate; 2 bay leaves; 100 g / 1 cup (ideally roasted*) walnuts, chopped; 225 g / 1½ cups cooked firm lentils; 10 ml / 2 tsp quality balsamic vinegar; 5 ml / 1 tsp date syrup or sugar (optional); 1 kg / 35 oz celeriac; a head of roasted garlic*; 2 tbsp white miso; approx. 80 ml / 1/3 cup plant milk; 2 tbsp extra virgin olive oil or vegan butter; 120 ml / ½ cup vegan cream; 1/3 tsp freshly grated nutmeg, adjust to taste; fine sea salt, more to taste