Directions

• Cook and stir the cut-up smoked sausage in a large skillet over medium heat for 6 to 8 minutes, until heated through and beginning to brown. Remove the sausage from the skillet, and set aside.
• Fill a pan with lightly salted water, bring to a boil over medium-high heat, stir in the macaroni, and return to a boil. Cook, stirring occasionally, until the pasta has cooked through but is still firm to the bite, about 8 minutes. Drain well.
• Preheat an oven to 350 degrees F.
• Grease a 9x13 baking dish.
• Melt the butter in the skillet over medium-low heat, and cook and stir the chopped onion for about 5 minutes, until translucent. 
• Whisk in the flour, stirring constantly to avoid lumps.
• Cook and stir the butter, onion and flour for 2 to 3 minutes to make a roux, and remove from the heat. 
• Whisk in the milk a little at a time, stirring constantly, until all the milk has been incorporated, and return to low heat.
• Bring the sauce to a simmer, and cook over low heat for about 2 minutes, stirring constantly, to finish cooking the flour. 
• Whisk in the Cheddar cheese, a few cubes at a time, until all the cheese has been incorporated and the sauce is hot and smooth.
• Pour the macaroni into the cheese sauce, and stir to combine.
• Stir in the cooked sausage, salt, and pepper.
• Spoon the macaroni mixture into the prepared baking dish, and sprinkle the bread crumbs over the top. 
• Bake for about 20 minutes in the preheated oven, until the crumbs are brown and the casserole is bubbling.
• Let stand for 15 minutes after baking, to set before serving.