Make crust: Whisk together the flour and salt in a large bowl. Sprinkle bits of butter over dough and using a pastry blender, cut it in until the mixture resembles coarse meal, with the biggest pieces of butter the size of tiny peas. In a small bowl, whisk together the sour cream, lemon juice and water and add this to the butter-flour mixture. With your fingertips or a wooden spoon, mix in the liquid until large lumps form. Gather the lumps into a slightly flattened ball. Wrap with plastic wrap or a large piece of parchment paper, and chill in fridge for 1 to 2 hours.

Make filling: Spread the zucchini out over several layers of paper towels. Sprinkle with 1 teaspoon salt and let drain for 30 minutes; blot the tops of the zucchini dry with paper towels before using. In a small bowl, whisk the olive oil and the garlic together; set aside. In a separate bowl, mix the ricotta, parmesan, mozzarella, and 1 teaspoon of the garlicky olive oil together and season with salt and pepper to taste.

Assemble galette: Heat oven to 400 degrees. On a floured work surface, roll the dough out into a 12-inch round. Transfer to an ungreased baking sheet (though if you line it with parchment paper, even the one you wrapped the dough in, it will be easier to transfer it to a plate later). Spread the ricotta mixture evenly over the bottom of the galette dough, leaving a 2-inch border. Shingle the zucchini, slightly overlapped, on top of the ricotta in concentric circles, starting at the outside edge and working your way into the center. Drizzle the remaining tablespoon of the garlic and olive oil mixture evenly over the zucchini. Season zucchini with black pepper. Fold the border over the filling, pleating the edge to make it fit. The center will be open. Brush crust with egg yolk glaze.

Bake the galette: Until the cheese is puffed, the zucchini is slightly wilted and the galette is golden brown, 30 to 40 minutes. Remove from the oven, sprinkle with basil, let stand for 5 minutes, then slide the galette onto a serving plate. Cut into wedges and serve hot, warm or at room temperature.