Cook pasta in salted water according to package instructions.
Preheat the oven to 375°F and prepare a baking dish with buttered flavor Pam spray.
In a large, heavy skillet, melt butter over low heat. Add flour and stir with a whisk.
Add onion and cook for another 2 minutes. Pour in milk and chicken broth, whisking continuously and raising heat to medium-high until it comes to a boil. Season with salt and pepper to taste.
Remove from heat and add cheese gradually (1/4 cup at a time), stirring well until cheese is melted. Add cooked pasta and baby spinach, and mix well.
Pour the mixture into the prepared 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. Enjoy!