Okay... here it is, my super-secret recipe for the absolute best mac and cheese! My family has begged for this recipe for years, I guess it's time to share! If you're counting calories, this is NOT the recipe for you, but it is definitely worth it! Makes a wonderful potluck dish as it does make a large amount.
Boil macaroni in salted water per package directions to desired doneness. Allow pasta to rest in water until needed. Preheat oven to 400 degrees. Grease a 9"x13" glass dish and set aside.
Fry bacon until crispy, remove to plate to cool and reserve drippings. Heat bacon drippings in pan, add 1/2 chopped onion, 1 1/2 tsp rosemary, and 1 tbsp Italian seasoning. Saute until onions are caramelized. Remove to small bowl (be sure to include bacon drippings). Crumble bacon and add to this mixture.
In medium sized saucepan, melt butter. Add remaining chopped onion, rosemary, and Italian seasoning. Saute until onions caramelize. Reduce heat to medium/low. Carefully pour in heavy cream and 1 cup of chicken broth, heat until boiling. Stir constantly, it will scorch easily. Add cheeses and stir until smooth and cheese is melted. Pour in bacon mixture and mix well. In small cup, mix 1/2 c. chicken broth with 1 TBSP corn starch. Blend into cheese mixture and heat until thickened. Remove from heat.
Drain macaroni and pour into prepared dish. Pour cheese mixture over and mix well to coat all macaroni.
Mix breadcrumbs with grated Parmesan cheese and garlic salt. Sprinkle evenly over macaroni and bake until bubbly and breadcrumbs are toasted, about 10-15 minutes. Serve immediately, store leftovers (if any!) in refrigerator.