Cook linguine according to package directions. Drain and rinse with cold water; set aside.
Melt butter in a medium saucepan over medium high heat. Stir in flour. Add milk; bring to a boil, stirring constantly until thick and smooth. Add cheese, poppy seeds, onion salt and pepper. Remove from heat.
Break the egg into a small bowl; beat with a fork. Stir a small amount of the hot sauce into the beaten egg; then stir egg mixture into remaining sauce.
Combine cooked linguine, sauce, broccoli and cashews in a large bowl. Transfer to a greased 2-quart baking dish. Sprinkle with 1/4 cup shredded Parmesan cheese. Bake for 35 minutes.