Preheat oven to 180°C. Grease and line a 20cm square tin with baking paper. Bring 500ml water to the boil in a saucepan. Add polenta and stir until water is absorbed. Spread into a 20cm square tin lined with baking paper. Allow to set.
Combine ricotta, sun-dried tomatoes, parmesan and thyme leaves. Season. Brush both sides of mushrooms with 2 tablespoons of the oil. Spoon ricotta mixture into mushroom cups and put onto an oven tray. Roast for 12-15 minutes or until mushrooms are soft.
Combine remaining oil, vinegar and garlic. Toss rocket in half of the dressing. Remove polenta from tin and cut into quarters. Cut each piece in half diagonally and put on serving plates.
Sprinkle rocket salad on top of polenta and top with a mushroom. Drizzle with remaining dressing, sprinkle with extra thyme and serve.