Preheat oven to 220°C fan forced. Place onions in a microwave-safe bowl. Cover with damp paper towel. Microwave on High (100%) for 3 minutes or until soft.
Heat the butter and oil in a 22cm (base) non-stick ovenproof frying pan (see tip) over medium-high heat. Add the onions and sugar. Cook for 5 minutes or until onions are light golden. Add the mushrooms, thyme and vinegar. Cook for 2 minutes or until mushrooms are tender. Season with salt and pepper.
Working quickly, lay the pastry over the mushroom mixture, tucking the edges under. Place the frying pan in the oven and bake for 20 minutes or until pastry is puffed and golden.
Turn the tart onto a large serving plate. Top with the rocket and serve.