Cook pasta in salted water according to package directions. Spray a baking dish with buttered flavor Pam.
Preheat oven to 375F°.
In a large, heavy skillet, melt butter. Add flour and cook over low heat stirring with a whisk.
Add onion and cook another 2 minutes. Add milk and chicken broth and continue whisking, raising heat to medium-high until it comes to a boil and becomes smooth and thick. Season with salt and pepper.
Once it becomes thick, remove from heat, add cheese 1/4 cup at a time and mix well until cheese is melted. Adjust salt and pepper to taste, add cooked macaroni and baby spinach.
Pour into baking dish. Top with grated cheese and breadcrumbs. Spray a little more oil on top.
Bake for 15-20 minutes, then broil for a few minutes to get the breadcrumbs golden.