01 - 
                Preheat your oven to 400°F (200°C). Wash the zucchini thoroughly. Trim off the ends, then slice each zucchini in half lengthwise.
              
              
              
                02 - 
                Using a spoon, carefully scoop out the flesh from the centers of each zucchini half, leaving about a 1/4 to 1/2-inch thick shell. Roughly chop 1 cup of the scooped-out flesh and set it aside.
              
              
              
                03 - 
                Place the hollowed-out zucchini boats cut-side up on a baking dish. Lightly brush them with olive oil and sprinkle with salt and pepper. Bake for 10-15 minutes, then remove and set aside.
              
              
              
                04 - 
                Heat 1 tablespoon olive oil in a skillet over medium heat. Add the chopped yellow onion and cook until softened, about 5-7 minutes. Stir in the minced garlic and cook for another minute until fragrant.
              
              
              
                05 - 
                Add the lean ground beef to the skillet, breaking it apart with a spoon. Cook until browned, about 8-10 minutes. Drain off excess fat.
              
              
              
                06 - 
                Stir in the chopped zucchini flesh and cook for 3-5 minutes. Add diced tomatoes, tomato sauce, Italian seasoning, salt, and black pepper. Bring to a simmer, reduce heat to low, and cook for 10-15 minutes.
              
              
              
                07 - 
                If using, stir in cooked rice or quinoa. Remove from heat and mix in 1/2 cup shredded mozzarella cheese until combined.
              
              
              
                08 - 
                Evenly spoon the filling into the pre-baked zucchini boats, packing gently but generously.
              
              
              
                09 - 
                Sprinkle the remaining 1/2 cup shredded mozzarella and 1/4 cup Parmesan cheese evenly over the stuffed zucchini boats.
              
              
              
                10 - 
                Bake in the preheated oven at 400°F (200°C) for 20-25 minutes, or until the zucchini is tender, the filling is hot, and the cheese is melted and golden brown.
              
              
              
                11 - 
                Remove from the oven and let rest 5-10 minutes before serving. Garnish with fresh parsley or basil if desired.