Pour the warm water into a bowl and add the yeast. Whisk it together and leave for five minutes. Next, add salt, flour, and butter, and mix thoroughly.
Transfer the mixture onto a floured work surface and knead with your hands for three to five minutes until you get a smooth and elastic dough. Transfer the dough to a bowl greased with olive oil, cover with plastic wrap and leave to rise for 1 to 2 hours.
For the Filling
Place a pan over medium heat. Once it warms up, add sausage and cook for four to six minutes until it is no longer pink.
Add mushrooms and onions to the pan and season with salt and black pepper.
Cook for five more minutes until the onions are tender.
Once done, remove from heat and set aside to cool.
Assembly
Once the dough is ready, remove it from the bowl, roll it on the flour-dusted work surface. Then divide it into four equal portions, and then roll each dough into a ¼" thick circular sheet using a rolling pin.
Spread marinara sauce on each sheet, put some meat filling in the center, add mozzarella cheese, Canadian bacon, and pepperoni, then fold the dough in half and seal the edges by pinching with fingers.
Brush egg on the surface of the calzones and arrange on the parchment-lined baking sheet. Transfer the baking sheet to the preheated (425 degree F) oven, and bake for 15 to 20 minutes until golden brown.
Once ready, remove the calzonis and garnish with some marinara sauce and fresh basil.
Add in any meat of your choosing such as ground pork, beef, or even chopped chicken breast.
Before storing these as leftovers, cool the calzones completely, then wrap them in tin foil, and place them in the Ziploc bag. These can be kept in the freezer for up to three months.