Polenta lasagne with mushroom ragù

Name: Polenta lasagne with mushroom ragù
Ingredients: 3 tbsp extra virgin olive oil; ½ onion, finely chopped; 1 thyme sprig, leaves picked; 500g/1lb 2oz chestnut mushrooms, sliced; 700ml/1¼ pint passata; sea salt and freshly ground black pepper; pinch sea salt; 200g/7oz quick-cook polenta; 25g/1oz butter, plus extra for greasing; 125g/4½oz mozzarella, drained and roughly chopped; 60g/2¼oz grated vegetarian hard cheese, plus extra to garnish; handful rocket; 1 radicchio, leaves seperated; 1 castelfranco, leaves separated; 1 lemon, juice only; 2 tbsp extra virgin olive oil