Instructions

1. Preheat oven to 350°. Fit pastry into a 9" × 12" baking sheet, pressing it against sides. Score around bottom inner edge of pastry (beside crease where bottom meets sides), being careful to not cut all the way through, with a paring knife. Prick bottom of pastry all over with a fork, line with a sheet of parchment paper that fits in bottom only, and fill with pie weights or dry beans. Bake until edge of crust begins to puff and color, about 25 minutes. Remove weights and paper. Bake until bottom is golden, 6–8 minutes more. Let crust cool.
2. Grate 4 of the zucchini on large holes of a box grater into a large bowl. Add 1 tbsp. of salt, toss well, and set aside to let weep for 30 minutes. Transfer to a clean kitchen towel and wring throughly to remove moisture.
3. Meanwhile, slice remaining zucchini into 1⁄4"-thick rounds. Working in batches, blanch rounds in a large pot of boiling salted water for 1 minute. Drain and spread out on a paper towel–lined sheet pan; set aside.
4. Heat butter in a large skillet over medium heat. (Spoon out 1 tbsp. and reserve.) Add onions and cook until soft, 5–6 minutes. Add grated zucchini and cook, stirring often, until just beginning to brown, 5–7 minutes. Transfer to a large bowl; let cool.
5. Stir tomatoes, half of the feta, ricotta, basil, and salt and pepper to taste into zucchini mixture. Stir in egg and spread mixture evenly in crust. Arrange zucchini rounds, slightly overlapping in rows, like tiles, on top. Bake for 15 minutes, then brush top with reserved butter. Continue to bake until crust is deep golden, 10 minutes more. Let cool to room temperature, then sprinkle remaining feta over top. Cut tart into squares.