Heat the oven to 375 F.

Spray a shallow 2 1/2-quart baking dish with cooking oil spray.

Heat the olive oil in a large skillet over medium heat. Add the ground beef and cook for 6 minutes, stirring frequently. Drain and discard excess fat, if desired.

To the browned beef, add the onion, bell pepper, and garlic and continue to cook for 2 to 3 minutes longer, or until the onion is softened. Add the tomatoes, tomato sauce, bay leaf, parsley, basil, kosher salt, and black pepper, along with the crushed red pepper flakes, if using. Bring to a simmer. Reduce the heat to low and simmer the sauce for 15 minutes.

Remove the bay leaf and stir the Parmesan cheese into the sauce.

Meanwhile, cook the spaghetti in boiling salted water following the package directions. Drain well.

Combine the drained spaghetti with the sauce mixture and toss to blend thoroughly.

Turn the spaghetti mixture into the prepared baking pan and the sprinkle with the shredded mozzarella cheese evenly over the top.

Bake for about 20 minutes, or until the casserole is bubbling around the edges and the cheese is melted.

Serve the casserole with garlic bread and pass more Parmesan cheese at the table.