1. Position a rack in the middle of the oven. Preheat the oven to 375°F. Have ready a 6-cup capacity baking dish.

2. In a large saucepan that has at least a 5-quart capacity, melt the butter over medium heat for about 1 minute. Add the onion and cook until it softens, about 5 minutes, stirring often. Rinse the spinach and add it to the saucepan along with any water that clings to the leaves. Cover and cook until the spinach shrinks and darkens in color, about 5 minutes. Uncover the pan and continue cooking until all of the moisture has evaporated, about 5 minutes. Transfer the spinach mixture to a plate to cool slightly. Drain off any liquid that accumulates. As soon as the spinach is cool enough to handle, squeeze it dry and use a large knife to chop it coarsely.

3. In a large bowl, stir the eggs, ricotta, and Parmesan cheese together to combine them. Add the spinach, salt, and pepper and stir to blend the spinach evenly into the filling. Use a rubber spatula to scrape all of the filling into the baking dish.

4. Lightly flour the rolling surface and rolling pin. Roll the crust dough to a shape that is 4 inches larger than the top of the baking dish. Roll the dough around the rolling pin and unroll it over the spinach filling. Fold ½ inch of the edge of the dough under to form a smooth edge, then roll the edges to make a rope of crust, or use the tines of a fork to press the dough firmly onto the rim of the baking dish.

5. Bake for about 40 minutes, or until the crust is lightly browned and the filling is sizzling gently. Use a large spoon to cut down through the crust and scoop out servings of crust and filling.