Preheat the oven to 350 degrees.
Slice the zucchini in half lengthwise. Use a spoon to scoop out the center down to about 1/2" thickness, reserving about 1/2 cup. Cut up the reserved zucchini it if it's not too seedy. If it is, I recommend using another zucchini for the filling (this is what I did) and cutting it into 1/2" dice. Put the two halves of the zucchini into a large baking dish.
Dice the onion, tomatoes, mushrooms, bell peppers and eggplant into 1/2" pieces, Mince the garlic.
Remove the sausage from the casings and break it apart into small pieces. Cook the sausage all the way through in a pan. Set aside to cool.
Heat a large pan (I used a non-stick wok) and add 2 Tbsp of olive oil. Cook the onions until translucent. Add in the eggplant, mushrooms, zucchini and bell peppers.
Cook until they start to get soft and then add in the garlic and cook another 2-3 minutes.
Remove from the heat and allow to cool until you can touch it (we're going to add in the egg next and you don't want it to cook as soon as it touches the hot vegetables).
In a large bowl, mix together the sausage and the vegetables. Mix in the beaten egg, Parmesan cheese, fresh breadcrumbs and 2 Tbsp. fresh basil to bind it all together. Scoop it into the zucchini halves, mounding up the filling.
Cover with foil and seal the edges around the baking dish (this will keep it moist and help steam cook the zucchini). Bake for 45 minutes or until the zucchini is tender (but not mushy!). Remove the foil and turn the oven to broil. Sprinkle the top with grated Parmesan and place it under the broiler for a few minutes until lightly browned. Remove and garnish with more fresh basil.