Preheat your oven to 400°F (200°C) and line a baking sheet with parchment paper.
In a large mixing bowl, combine the flour, baking powder, salt, garlic powder, and Italian seasoning. Mix well.
In a separate bowl, mix together the ricotta cheese, mozzarella cheese, cooked sausage, spinach, egg, olive oil, Parmesan cheese, black pepper, and red pepper flakes until well combined.
Gradually add the dry ingredients to the cheese mixture, stirring until a dough forms. If the dough is too sticky, add a little more flour as needed.
Turn the dough out onto a floured surface and knead gently for about 1 minute until smooth.
Divide the dough into 4 equal portions and roll each portion into a circle about 6 inches in diameter.
Spoon a generous amount of the filling onto one half of each dough circle, leaving a small border around the edge.
Fold the other half of the dough over the filling and press the edges together to seal. Use a fork to crimp the edges for an extra seal.
Place the calzones on the prepared baking sheet and brush the tops with a little olive oil.
Bake in the preheated oven for 20-25 minutes, or until golden brown.
Allow to cool for a few minutes before serving.