650g/1lb 7oz Portobello mushrooms, cut into 3cm/1¼in wedges
2 tsp lemon juice
1 tsp roughly chopped sage leaves
200g/7oz small shallots, peeled and left whole
1 tsp roughly chopped, tarragon leaves
salt and freshly ground black pepper
150g/5½oz ready-cooked and peeled chestnuts, broken in half
2 garlic cloves, crushed
4 tbsp olive oil, plus 2 tsp to serve