Step 1

Make the dough. Combine the water and yeast in a large bowl, then wait a few seconds until the yeast dissolves. Add the flour and salt and mix with your hands or a spatula until it forms a shaggy dough.

Step 2

Knead the dough. Turn the dough and any loose flour from the bowl out onto a clean work surface. Knead until all the flour is incorporated and the dough is smooth and elastic, 6 to 8 minutes. The dough will feel moist and a little tacky. If the dough sticks to your hands significantly, knead in more flour, 1 tablespoon at a time, until smooth.

Step 3

Check the dough for windowpane. Tear off a small piece of dough and use your fingertips to gently stretch it into a square. If the dough stretches thinly enough that you can see light passing through it without tearing right away, you have developed enough gluten to pass the windowpane test. If not, keep kneading and check again after a few minutes. Incorporate that small piece back into the dough and knead into a ball.

Step 4

Let the dough rise until doubled in size. Coat the bowl and the top of the dough with the olive oil, then place the dough in the bowl. Cover with a clean kitchen towel and and let rise until doubled in size, about 1 1/2 hours. (Alternatively, refrigerate the dough in a large, airtight container for up to 3 days. Let sit at room temperature for 1 hour before continuing with the calzone.)

Step 5

Heat the oven. Arrange a rack in the lower third of the oven (place a pizza stone on the rack if using), and heat to 450°F. Line a baking sheet with parchment paper; set aside.

Step 6

Make the spinach-ricotta filling. Heat the oil in a large skillet over medium-high heat until shimmering. Add the onion and garlic and cook, stirring occasionally, until softened and golden brown, 6 to 8 minutes. Add in the spinach (in batches if needed) and toss until wilted, about 1 minute. Remove from the heat and let cool for 2 to 3 minutes. Add the ricotta and mozzarella and stir to combine; set aside.

Step 7

Divide the dough. Cut dough into 8 (2-ounce) pieces.

Step 8

Roll out the dough. Working with one piece of dough at a time, press each piece into a flat disc, then use a rolling pin to roll into a 6- to 7-inch round. Be sure to maintain an even thickness of the dough, as the calzone will burst if the dough is too thin. If the dough shrinks back and is difficult to roll, let it rest for 5 minutes and try again. Repeat with the remaining pieces of dough.

Step 9

Fill the calzone with tomato sauce and spinach-ricotta filling. Spread 1 tablespoon of the tomato sauce on the bottom half of each calzone dough round, leaving a 1-inch border around the bottom edge of the dough. Spoon 2 tablespoons of the filling on top of the sauce. Do not overfill the calzone.

Step 10

Fold and crimp the edges of the dough. Fold the dough over the filling, making sure not to pull the dough too tightly. Press the edges tightly to seal and roll the bottom edge up and over the top, crimping the dough to seal. Transfer the calzones to the baking sheet.

Step 11

Brush the calzones with egg wash and cut steam vents. Beat the egg with 1 tablespoon of water with a fork in a small bowl until the egg is broken up. Brush each calzone lightly with the egg wash. Using a sharp knife, cut 2 to 3 steam vents on the top of each calzone.

Step 12

Bake the calzones. Bake until the calzones are golden brown and the filling bubbles, 15 to 20 minutes. It is likely that at least 1 calzone will burst, despite all efforts to prevent it. Let the calzones cool for a few minutes before serving.