Step 1: Preheat the Oven

1. Preheat your oven to 375°F (190°C).
2. Grease a large baking dish or sheet with cooking spray or a little olive oil and set aside.

Step 2: Prepare the Zucchini Boats

1. Wash the zucchinis and trim off the ends. Slice each zucchini lengthwise and use a spoon or melon baller to scoop out the center flesh, leaving about ¼ inch around the edges to form “boats.” Save the scooped-out zucchini flesh for later use.
2. Place the hollowed-out zucchinis cut side up in the prepared baking dish. Drizzle with olive oil and sprinkle with salt and pepper.

Step 3: Cook the Ground Beef Filling

1. Heat a large skillet over medium heat. Add the ground beef and cook, breaking it up with a spatula, until browned and fully cooked through, about 5-7 minutes. Drain any excess grease.
2. Add the chopped onion and minced garlic to the skillet with the beef, cooking for another 2-3 minutes until the onion becomes translucent and the garlic is fragrant.
3. Stir in the chopped zucchini flesh (from Step 2), and cook for an additional 3-4 minutes, allowing the mixture to soften and combine. Season with Italian seasoning, red pepper flakes (if using), salt, and pepper to taste.
4. Stir in 1 cup of cooked rice and ½ cup of marinara or tomato sauce. Mix everything together and cook for 2-3 minutes until heated through and well combined.

Step 4: Stuff the Zucchini Boats

1. Spoon the hamburger and rice mixture into each zucchini boat, filling them generously.
2. Top each stuffed zucchini with a spoonful of the remaining marinara sauce, followed by a sprinkle of shredded cheddar cheese and grated Parmesan cheese.

Step 5: Bake the Zucchini Boats

1. Cover the baking dish loosely with aluminum foil and bake in the preheated oven for 20 minutes.
2. After 20 minutes, remove the foil and bake for an additional 10-15 minutes until the cheese is melted, bubbly, and slightly golden. The zucchinis should be tender but not mushy.

Step 6: Garnish and Serve

1. Once the zucchini boats are done baking, remove them from the oven and allow them to cool for a few minutes before serving.
2. Garnish with freshly chopped parsley or basil for a pop of color and extra flavor. Sprinkle with extra Parmesan cheese, if desired.