Heat a large skillet or pot to high heat. Add olive oil and onion. Cook onion until golden. Add ground beef, garlic and spices. Cook ground beef for 3-5 minutes or until fully browned. Add 1/2 cup marinara sauce and potatoes. Cook for 5 minutes.
While the ground beef is cooking, cut zucchini lengthwise in half; scoop out centers, leaving 1/4-inch-thick shells. Combine 1/2 cup water and 1 cup marinara sauce in a large 13x9 casserole dish. Place zucchini boats in sauce, divide beef mixture evenly into zucchini boats, top with cheese.
Bake 15 minutes or until cheese is fully melted. Top with fresh diced tomatoes, avocado, and cilantro.