Preheat oven to broil. Place sausages on a parchment-lined baking sheet. Broil for about 15 minutes. Set at room temperature a few minutes before cutting into slices. Set aside.
Cook macaroni according to package instructions. Meanwhile, melt butter in a large saucepan over medium heat. Add onion and fry until translucent. Add flour and cook over low heat for 1 minute, being careful to not let it brown.
Add in half the milk and stir until smooth. Add remaining milk, stirring continuously, until bubbles begin to appear at surface. Add cheese and mustard; stir only until combined. Season with salt and pepper. Add reserved spinach and sliced sausage to sauce and then turn off heat. Add macaroni to sauce, mix well, and transfer to a 33 x 23-cm (13 x 9-in.) rectangular baking dish. Top with breadcrumbs. Broil for 5 minutes or until breadcrumbs are golden brown.