1Preheat 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.
2Heat 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.
3Working 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.
4Turn the tart onto a large serving plate. Top with the rocket and serve.
Tip: Frying pans suitable for both the stove top and oven are very handy. It’s important to check the material the handle is made from. If you don’t have an ovenproof frying pan, transfer the mixture at the end of step 2 into a lightly greased 22cm (base) cake pan. Complete steps 3 and 4.