Preheat your oven to 400 degrees Fahrenheit and prepare a baking sheet lined with parchment paper. Set aside.
Add the cooked Italian sausage to a bowl (drain it first!), along with the chopped mushrooms, marinara sauce, 1/2 cup shredded cheese and oregano. Mix well and set aside.
Slice the 3 zucchini in half, lengthwise, and using a spoon hollow out the inside parts of the zucchini where the seeds are. Discard the insides.
Divide the sausage mixture between the 6 hollowed out zucchini halves and press the mixture down into the zucchini firmly.
Top each of the halves with a generous amount of shredded mozzarella cheese and place the zucchini halves on the prepared baking sheet.
Bake at 400 degrees Fahrenheit, uncovered, for about 20 minutes or until the zucchini is soft and the cheese is golden brown and bubbly.
Serve immediately, or store in airtight containers in the fridge for up to 3 days to reheat for leftovers or healthy lunches.