Bring a large pot of generously salted water to a boil. Cook the pasta according to package directions until al dente. Reserve 1/2 cup pasta water before draining. Drain and set aside.

While the water heats, warm the olive oil in a large deep skillet over medium-high heat. Add the onion and cook, stirring occasionally, for 3 to 4 minutes until it softens and turns translucent. Add the garlic and cook for another 30 seconds until fragrant.

Add the ground beef to the skillet. Break it apart with a wooden spoon and cook for 5 to 6 minutes, until no pink remains. Tilt the pan and spoon out any excess fat, leaving about a tablespoon behind. Do not skip this step, draining the fat keeps the sauce from being greasy.

Add the diced zucchini to the skillet with the beef. Cook, stirring occasionally, for 2 to 3 minutes so the zucchini picks up a little color from the pan. Then pour in the marinara sauce. Add the Italian seasoning, salt, pepper, and red pepper flakes if using. Stir well to combine.

Reduce the heat to medium and let the sauce simmer for 7 to 8 minutes, until the zucchini is tender but not mushy when you press a piece with a spoon. Taste the sauce and adjust the seasoning.

Add the drained pasta to the skillet and toss to coat everything evenly. If the sauce looks too thick, stir in some of the reserved pasta water a splash at a time until it reaches a consistency you like. Serve in bowls with grated Parmesan on top and garlic bread on the side.