Onion and artichoke tart

Name: Onion and artichoke tart
Ingredients: 4 tbsp olive oil; 4 large onions, sliced into thick wedges; 1 tsp light brown sugar; 1 tsp balsamic vinegar; 400g/14oz mixed mushrooms, such as chestnut, shiitake, button, thickly sliced; 375g packet ready-rolled puff pastry; 1 large egg, beaten; 75ml/2½fl oz double cream; 2 tsp chopped fresh sage; 1 tsp chopped fresh thyme leaves; 175g/6oz chargrilled artichoke, drained and cut into large pieces; 100g/3½oz mature cheddar, grated; salt and freshly ground black pepper; fresh salad, to serve