Preheat the oven to 375 degrees. Line a rimmed baking sheet with parchment paper or foil.

Slice the zucchinis in half lengthwise. Trim the ends if needed. Scoop out the insides using a spoon leaving a 1/2 -inch border. Place them on the prepared baking sheet.

Heat the olive oil in a large skillet over medium-high heat. Add the diced onions, cook, stirring often until softened, about 5 minutes. Add the garlic, cook until fragrant, about 1 minute. Add sausage or ground beef, cook until no longer pink, about 5 minutes. Season with salt and pepper. Add the marinara and crushed red pepper flakes, stir to combine. Reduce the heat to low and simmer, stirring occasionally.

TO ASSEMBLE: Spread the ricotta on the bottom of each zucchini boat, then spoon the meat sauce over the ricotta layer. Sprinkle the tops with mozzarella cheese.

Bake uncovered until the zucchini is tender and the cheese is melty, about 15 minutes.