Preheat oven to 350°. Score zucchini (like you’re dicing an avocado) and scoop out insides.
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 more. Add ground beef and cook until no longer pink, 5 minutes. Season with salt and pepper. Add marinara and stir until combined. Add crushed red pepper flakes.
Spread ricotta on bottom of each zucchini boat, then spoon beef sauce over ricotta layer and sprinkle with mozzarella.
Bake until zucchini is tender and cheese is melty, 15 minutes.
Garnish with parsley before serving
