Cook the onion in the olive oil on medium heat for 5 minutes, stirring occasionally. Add the zucchini and continue cooking and stirring just until the zucchini softens a little, about 6 to 8 minutes. Stir in the seasonings and remove from heat. Allow to cool before assembling the tart.
Grease a 9 or 10 inch round baking pan. Place the zucchini slices in concentric circles on the bottom of the pan. Sprinkle with the onion. Break the goat cheese into small pieces and distribute evenly over the vegetables.
Cover the filling ingredients with the pastry, tucking in the edges around the vegetables. Bake at 400° F for 20 minutes or until golden brown. Remove from oven and allow to cool for 5 to 10 minutes before flipping onto a serving plate.