Store-bought dough, plenty of fresh spinach, and meatballs and sauce reserved from 30-Minute Spaghetti and Meatballs make this an easy, winning meal.
Preheat oven to 450 degrees. Lightly oil a rimmed baking sheet. In a large saucepan, bring 1/2 cup water to a simmer. Add spinach and season with salt; cover and cook until wilted, about 2 minutes. Transfer spinach to a fine-mesh sieve and drain, pressing with a rubber spatula to release as much liquid as possible. Transfer spinach to a cutting board and roughly chop.
On a lightly floured work surface, roll each piece of dough into an 8-inch round. In a medium bowl, stir together spinach, reserved meatballs and sauce, cheese, and red-pepper flakes (if using). Season with salt and pepper. With a slotted spoon, divide mixture among bottom halves of dough rounds, leaving a 1-inch border. Fold top half of each round over filling and roll and pinch edges to seal. Carefully transfer calzones to baking sheet. Brush tops lightly with oil and season with salt and pepper. Bake until calzones are golden brown, 15 to 18 minutes.