In a large saute pan, heat the olive oil over medium heat. Add the leeks and onion and cook, stirring occasionally, until wilted, 5 minutes. Add the garlic, stir and cook until fragrant, 1 minute. Add the spinach and cook until wilted and tender, 2 minutes. Remove from the heat, taste and season with salt and pepper as needed; let cool.
Meanwhile, in a large bowl, whisk the ricotta, 2 eggs, the thyme and 1/2 teaspoon salt. Fold the cooled vegetable mixture and the olives into the ricotta mixture.
Preheat the oven to 375 degrees F. On a lightly floured surface, roll out the pastry until it's 12 inches square. Using a 12-inch plate or just eyeballing it, cut the dough into a circle (knead the scraps into a ball and save for another use-cheddar straws, say). Transfer the dough to a parchment-lined baking sheet. Spoon the tart filling into the center, leaving a 2-inch border of pastry. Lift the pastry edges and fold over the filling, creasing the dough as needed and leaving the filling exposed in the middle. In a small bowl, whisk the remaining egg with 1 tablespoon water and brush a thin layer of the egg wash on the exposed pastry, taking care not to let egg run down the sides to the pan.
Bake until the pastry is golden brown, about 45 minutes. Remove from the oven and let rest 10 minutes before serving.