Preheat your oven to 400°F (200°C). Line a large baking sheet with parchment paper. Pat the chicken breasts dry and season all over with the Creole seasoning.
Heat the olive oil in a large skillet over medium-high heat. Add the chicken and cook for 5-7 minutes per side, or until golden brown and cooked through. The internal temperature should reach 165°F (74°C) on an instant-read thermometer. Transfer the chicken to a cutting board to rest for 5 minutes, then chop into small, bite-sized pieces.
Reduce the heat to medium and add the butter to the same skillet. Once melted, add the minced garlic and cook for about 1 minute until fragrant. Whisk in the flour and cook for another minute, stirring constantly.
Slowly pour in the chicken broth while whisking to avoid lumps. Then, gradually whisk in the milk until the sauce is smooth. Add the dried basil, and season with salt and pepper. Let the sauce simmer for 2-3 minutes, stirring occasionally, until it has thickened enough to coat the back of a spoon. Remove from heat.
Unroll the pizza dough onto a lightly floured surface. Cut the dough into 4 equal rectangles. Gently stretch each piece into a rough 7-inch circle.
To assemble, spread a quarter of the cream sauce onto one half of each dough circle, leaving a 1/2-inch border around the edge. Top the sauce with a quarter of the chopped chicken, spinach, and shredded cheese.
Fold the empty half of the dough over the filling to create a half-moon shape. Press the edges together firmly and crimp with a fork to seal. Brush the top of each calzone with the egg wash and cut two small slits in the top to allow steam to escape.
Transfer the calzones to the prepared baking sheet. Bake for 12-15 minutes, or until the crust is deep golden brown and the filling is bubbly.
Let the calzones rest on the baking sheet for 5 minutes before serving. This helps the filling set and prevents burns.