Preheat oven to 350 degrees F. Score zucchini (like you’re dicing an avocado) and scoop out insides into a large bowl. In a large skillet over medium-high heat, heat oil. Add onion and cook until soft, 5 minutes, then add garlic and cook until fragrant, 1 minute. Add ground beef and cook until no longer pink, 6 minutes more, then add marinara and stir until combined. Add crushed red pepper flakes. Stuff zucchini: Spread ricotta on bottom of each zucchini boat and season with salt and pepper. Spoon sauce over ricotta layer and sprinkle with mozzarella. Bake until zucchini is tender and cheese golden, 15 minutes. Garnish with parsley and serve.