In large bowl, toss zucchini with ½ tsp (2 mL) salt. Place in colander set over large bowl. Let stand 1 hr. (discard liquid in bowl and wipe bowl dry). On work surface, pat zucchini dry with paper towel. Put back into bowl, toss with thyme and lemon zest.
Meanwhile, in another bowl, whisk together flour, cornmeal and remaining ¼ tsp (1 mL) salt; drizzle with olive oil. With fingers, rub oil into flour mixture to moisten. Separate 1 egg (reserve the egg white for brushing later). Fork-beat the egg yolk with 3 tbsp (45 mL) water. Drizzle over flour mixture; stir with fork to form loose dough. Knead dough into a ball. Wrap with plastic and let rest 30 min. at room temperature.
Preheat oven to 400ºF (200ºC). Transfer dough to 9-in. (23-cm) tart pan with removable bottom. Press out dough to cover bottom and sides of pan. Using fork, prick bottom of tart shell about every 1 in. (2.5 cm). Line tart shell with foil. Fill with pie weights (such as dried beans). Bake 15 min. then remove foil and weights. Bake additional 5 to 10 min., or until tart shell is lightly golden. Cool until barely warm. Fork-beat reserved egg white and brush about 2 tsp (10 mL) over bottom of tart shell. Reduce oven temperature to 375ºF (190ºC).
Sprinkle crumbled goat cheese evenly in bottom of tart shell. Arrange zucchini slices in single layer of overlapping slices over top. Place tart pan on baking sheet lined with parchment or foil. In bowl, whisk remaining 2 eggs with cream. Slowly pour egg mixture into tart shell (it will flow under the zucchini slices) and let it spread to edges. Bake 25 to 30 min., or until filling begins to turn golden. Set on wire rack to cool completely. Serve slices at room temperature with green salad.
Tart can be made up to 1 day in advance and kept chilled. Serve chilled, at room temperature or warmed.