First, prepare the gluten-free pizza dough, as described in this recipe or make your own favorite pizza dough (for example, with wheat flour, if not gluten-free).
Heat oil in a pan/skillet over medium heat and add the onion. Fry for about 3-4 minutes, then add garlic, mushrooms, and peppers. Fry for a further 6-10 minutes until the water from the veggies has evaporated.
Add onion powder, ground oregano, salt and pepper to taste and turn off the heat. Set aside until ready to use.
Preheat oven to 480° Fahrenheit (250° Celsius). Line a large baking sheet with parchment paper. Divide the pizza dough into 4 equally sized pieces.
Roll out one piece of dough on a floured surface into a 6-7" (15-18 cm) circle, about 1/4" (0.6 cm) thick.
Spoon a little of the tomato sauce on one half of the dough, add the mushroom/veggie mixture, and vegan cheese. Make sure to leave at least 1/2" (1.25 cm) of space around the edge of the dough.
Carefully fold the other half of the dough over, and pinch the edges of the pocket with your fingers to seal (check pictures in the blog post above). Please note that gluten-free doughs can be quite brittle and tend to burst easily. So make sure to handle it carefully.
Repeat this step with the remaining dough. Use a spatula to transfer the pizza pockets to the prepared baking sheet. Optional: Brush tops with a mixture of 1/2 tbsp soy sauce, 1/2 tbsp water, a few drops of oil, and a pinch of smoked paprika and turmeric.
Bake the calzones for about 14-18 minutes. Let cool for 5 minutes, then serve with the remaining pizza sauce on the side. Please don't burn your mouth as the filling is very hot. Enjoy!