Preheat your oven to 400 degrees F. Grease the inside of a 9-inch x 13-inch baking dish. Set aside.
Cut 5 of the zucchini in half lengthwise and then use a spoon to gently hollow out each half to make boats. (see note). Put the hollowed zucchini cut side down into the 9x13 dish and bake for 20-30 minutes, or until tender.
Chop the insides of the zucchini and the one remaining zucchini into bite-sized pieces. Set aside.
While the zucchini cooks, prepare the filling by placing a large skillet over medium-high heat. Add the olive oil and once hot, add the onion and garlic. Cook until onion starts to soften, about 2-3 minutes.
Add the ground sausage, Italian seasoning, salt, and pepper. Cook until the sausage is browned and cooked through.
Add the bell pepper, chopped zucchini, and diced tomatoes to the skillet. Cook, stirring frequently until the veggies are tender and almost all of the liquid has evaporated, about 30 minutes. (see note)
Once the zucchini boats are tender, remove them from the oven and carefully flip them over so the cut side is up.
Use a spoon to add the filling to the zucchini boats. Don't be afraid to pile it up! Top each with mozzarella cheese.
Return the baking dish to the oven and bake for 15-20 minutes, or until the cheese is bubbly and everything is heated through.
Remove the zucchini from the oven and sprinkle with chopped fresh oregano if desired. Serve warm.
Notes
Be careful when scooping out the insides of the zucchini!. Be gentle so you don't break the zucchini boat.
It is very important to cook the filling until the liquid evaporates. Both the tomatoes and zucchini release a lot of liquid while cooking. If it doesn't boil away the filling in the finished zucchini boats will be very watery.
You can make these in advance up through step 8. Cover and chill in the fridge. When you're ready to eat, top each boat with cheese and bake at 400 degrees F until warmed through, about 30 minutes.