Preheat oven to 350 degrees.
Slice the zucchini lenthwise, taking off the top 1/3 of the zucchini. Scoop out the pulp leaving a sturdy shell. Chop the pulp and set aside.
Heat oil in a large skillet. Cook onions until translucent and soft, about 5 minutes. Add garlic and cook for 1 more minute. Add zucchini pulp and sautee on medium high for 3-4 minutes. Remove to a colander to drain.
In the same skillet add ground beef and sausage. Brown and break apart. Add to the colander.
In a large bowl, mix zucchini mixture, meat, egg, bread crumb, basil and a dash of salt and pepper.
Spoon the stuffing into the zucchini boats.
Pour the marinara into an ovenproof dish and put the zucchini boats on top of the Parmesan cheese.
Cover with foil and bake for 30 minutes. Uncover and bake another 20 minutes, until hot and bubbly. Serve immediately.