Cook pasta according to package directions; drain and set aside.
Preheat oven to 375 degrees F.
In a large skillet, melt butter over medium heat and whisk in flour. Add onion and cook for another minute.
Whisk in milk and chicken broth, and bring to a boil. Once thickened, remove from heat and mix in cheddar cheese.
Season with salt and pepper, as desired. Add cooked macaroni and spinach. Pour into a 9x13-inch baking dish sprayed with nonstick cooking spray.
Top with breadcrumbs and spray top with nonstick cooking spray. Bake, uncovered, for 15-20 minutes or until golden brown on top.