Cook pasta in boiling salted water until not quite "al dente". Add the cauliflower and bring back to a boil. Drain and rinse, then set aside.
Heat a large saucepan over medium heat. Coat lightly with cooking spray and add the sausage. Cook the sausage until it is golden brown, stirring frequently. Remove the sausage from the pan and set it aside.
Add the olive oil to the pan. Add the onion, garlic, and bell pepper. Cook stirring often until everything is soft and fragrant. Add 1/4 cup of the chicken stock to the pan. Stir to loosen any brown bits. Add the flour and cook and stir for 2 minutes.
Add the remaining stock stirring until smooth. Stir in Half & Half and cook until the mixture thickens and is bubbly. Remove the pan from the heat and add in the grated cheese, stirring until it melts.
Coat a rectangular baking dish with cooking spray. Combine the pasta, cauliflower, sausage, and cheese sauce in the pan. cover the pan with foil and bake for 30 minutes at 350 degrees.
Combine the melted butter, bread crumbs, and Parmesan. Remove the foil and top the casserole with the crumb mixture. Return to the oven for another 10 minutes. Turn oven to broil and allow the topping to brown.
Remove from the oven and allow to rest for 10 minutes before serving.