Begin by draining the tomatoes in sieve and cooking the other vegetables. Heat the olive oil on medium heat and stir in the onion. Cook for two minutes, then add the zucchini and mushrooms. Cook just long enough to tenderize the vegetables - they should still retain their shape - about 5 minutes. Season with thyme, salt and pepper. Remove from heat and allow to cool before you assemble the tart.
Layer the zucchini, mushrooms, and tomatoes in an attractive pattern on the bottom of a greased 9 or 10 inch baking pan. Sprinkle with the onion, then break the goat cheese into small pieces and evenly distribute it on top. Place the puff pastry on top and tuck in the edges. Prick in several places with a fork and bake at 400°F for 20 to 25 minutes.
Remove from the oven and allow to cool for several minutes before flipping the tart onto a serving dish.
Makes 6 servings.