Bring a large pot of salted water to a boil for the pasta.

Meanwhile, heat the olive oil in a large sauté pan (or Dutch oven) over medium heat. When the oil is hot and shimmering, add the onion and garlic and cook for about 5 minutes until the onion is very soft and the garlic is fragrant. (Be careful not to burn the onion and garlic! If they start to brown, turn down the heat immediately.) Add the ground chicken and salt and cook for about 6 minutes, stirring and breaking up the meat into small pieces with a spatula or until no longer pink. Add the crushed tomatoes, chicken broth, oregano, and red pepper flakes (if using). Gently simmer the sauce for 15 minutes to thicken it slightly and allow the flavors to combine. Stir in the basil and ¼ cup of the Parmesan, then add the spinach and cook just until wilted, about 1 minute. Stir in the balsamic. Taste the sauce and season with a little extra salt and crushed red pepper if needed.

Cook the pasta al dente according to the package directions and drain. (If using gluten-free pasta be sure to rinse with cold water.)

Add the pasta to the sauce and stir to combine.

Transfer the pasta to a medium baking dish and top with the mozzarella. Sprinkle with the bread crumbs and remaining Parmesan cheese.

Place the baking dish under the broiler until the cheese is melted and bubbling and the bread crumbs are nicely browned, about 1-2 minutes. (Keep a close eye on it to prevent burning!)

Garnish with extra basil if you like and serve warm.