Preheat your oven to 425°F. While the oven heats, prepare all your ingredients: mince the garlic finely, dice the onion into small pieces, thinly slice the mushrooms, and roughly chop the spinach. This prep work ensures you won't scramble once you start assembling the calzones.

Heat 2 tablespoons of olive oil in a skillet over medium-high heat. Add the diced onion and sauté for 2-3 minutes until softened, then add the minced garlic and cook for another 30 seconds until fragrant. Add the sliced mushrooms and cook for 3-4 minutes, stirring occasionally, until they release their moisture and begin to brown. Finally, add the chopped spinach and cook for 1-2 minutes until wilted. This sautéing step develops deeper flavors and removes excess moisture from the vegetables so your calzone filling won't be soggy.

Remove the skillet from heat and let the sautéed vegetables cool for 1-2 minutes. Stir in the cooked shredded chicken, feta cheese, dried oregano, and red pepper flakes until evenly distributed. Taste and adjust seasonings if needed. This mixing ensures all your flavors are balanced throughout the filling before assembly.

Remove the pizza dough from refrigeration and place it on a lightly floured surface. Cut the dough into 4 equal circles (roughly 5-6 inches each). I like to use a bowl or small plate as a guide to keep them uniform in size. Divide the filling mixture equally among the 4 circles, placing it on one half of each circle and leaving a half-inch border. Fold each circle in half to create a half-moon shape, then press and seal the edges firmly by crimping with a fork or pinching with your fingers—this prevents the filling from leaking during baking.

Place the sealed calzones on a lightly greased or parchment-lined baking sheet, spacing them about 2 inches apart. Using a pastry brush or your fingertips, lightly brush the top of each calzone with olive oil—this helps them brown evenly and creates a golden, crispy crust.

Place the baking sheet in the preheated 425°F oven and bake for 12-15 minutes, until the calzones are golden brown on top. They should sound slightly hollow when tapped lightly. Remove from the oven and let cool for 2-3 minutes before serving. Serve warm with marinara sauce on the side for dipping.