Melt butter over medium heat. As it starts to melt, whisk in the flour. Then whisk in the milk, chicken broth, Italian seasoning, salt and pepper. Increase to medium-high heat and bring to a boil.
Continue whisking frequently for 4-5 minutes or until mixture becomes thick. Then remove from heat and stir in both cheeses. Taste and add any salt or seasonings to taste.
Cut sun dried tomatoes into small pieces. To the skillet with the cheese sauce, fold in the sun dried tomatoes, cooked macaroni, spinach leaves, chicken, mushrooms and onions.