Preheat oven to 350 degrees. Lightly grease a 9x13 or 3.5 quart baking dish.
Cook pasta shells per package instructions until al dente. Set aside.
Steam broccoli and chop crowns into bite size pieces. Set aside. To do this quickly, I put a few tablespoons of a water in a microwave safe bowl, place the broccoli in the water, and then cover the bowl with a microwave safe plate. I heat it on high for about 2 minutes, then drain the broccoli.
Remove sausage from casings and crumble. Heat olive oil in small pan over medium heat. Add crumbled chicken sausage and cook until slightly browned. Remove and set aside.
In a large sauce pan over medium heat, melt 4 tbsp butter and add onion, seasoning salt, pepper, nutmeg and chili powder, stir together well.
Sprinkle flour over ingredients and saute for 1-2 minutes.
Whisk milk into mixture and bring to a boil. Turn down to low and allow to simmer for 5 minutes, until slightly thickened, whisking frequently.
Stir in cheese and worcestershire sauce. Once well combined, stir in pasta, chicken and broccoli.
Place mixture in prepared baking dish.
Melt 2 tbsp butter and mix well with panko, sage, thyme and rosemary. Spread evenly over mac and cheese.
Bake for 30 minutes.