Begin by preparing your zucchini. Wash the outside skin and then trim off the top where the stem is.
Slice in half lengthwise and then use a spoon to carefully scoop out the seeds and some of the meat of the zucchini. You can either discard this or save it to add to the filling.
Heat olive oil in a large skillet, and brown ground beef and diced onion over medium heat, breaking up the ground beef as you go.
Once the beef is cooked, add the reserved ¼ cup of marinara sauce, uncooked rice, water, zucchini insides, and spices. Simmer for about 10 minutes, until the rice has soaked up the liquid.
While the filling is cooking, preheat oven to 375.
In a 9x13 pan, add the remaining 2 cups of marinara sauce and place prepared zucchini on top, cut side up.
With a small spoon, scoop the filling into each zucchini boat. Depending on the size of your zucchini, you may have a little extra filling.
Top with grated parmesan and bake for 30-35 minutes.