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To make the crust: Whisk together the dry ingredients. Work in the butter until the mixture is crumbly. Drizzle in 5 tablespoons of water, stirring gently until everything is evenly moistened; add the final tablespoon of water if necessary to make a cohesive dough.
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Pat the dough into a disk, wrap, and refrigerate for 30 minutes.
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To make the filling: Preheat the oven to 425°F. Lightly grease (or line with parchment) two baking sheets.
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Place the zucchini slices on one pan and sprinkle with 1 1/2 teaspoons pizza seasoning.
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Place the tomato halves on the second pan and sprinkle with the remaining pizza seasoning.
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Roast the zucchini and tomatoes until tender, about 15 to 20 minutes for the zucchini and 10 to 15 minutes for the tomatoes.
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Remove the zucchini and tomatoes from the oven and allow to cool for 10 minutes.
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Combine the ricotta, salt, pepper, lemon zest, and egg until evenly blended. Set aside.
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To assemble the galette: On a lightly floured work surface, roll the dough into a 12" circle. Transfer the dough to a parchment-lined baking sheet.
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Spread the ricotta mixture over the dough, leaving a 2"-wide bare strip along the perimeter.
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Sprinkle half the Parmesan over the ricotta, then shingle the zucchini slices over the cheese and scatter the tomato halves on top.
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Fold the bare edges of the dough into the center.
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Brush the exposed edges of the crust with egg wash, and sprinkle the remaining Parmesan over the whole galette.
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Bake the galette for 25 to 30 minutes, until the crust is golden brown and the filling is bubbling.
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Remove the galette from the oven and allow it to cool for 5 to 10 minutes before serving.