Place noodles in a bowl of very hot water and let stand. Place 1/2 cup milk in a bowl, whisk in cornstarch and set aside. Microwave remaining milk at full power for 2 minutes.
Melt butter in a heavy saucepan over medium heat. Add onion and cook for 5 minutes. Add flour and whisk to make a paste. Gradually whisk in hot milk. Whisk cornstarch and milk into hot sauce. Bring to a boil, whisking; continue to whisk for 2 minutes. Remove from heat; stir in Cheddar, salt and pepper. Remove 1 cup cheese sauce and set aside. Let remaining sauce cool in pot.
Cook ground round over medium heat until browned. Transfer beef with a slotted spoon to cheese sauce in pot; stir in broccoli.
In a small bowl, mix bread crumbs, olive oil and parsley. Preheat oven to 400°F.
Butter a 9-inch square baking dish. Lay 1 noodle on a work surface; top with 1/2 cup beef mixture and roll up; transfer to dish. Repeat with remaining noodles. Spoon Cheddar sauce on rolls and sprinkle breadcrumb mixture on top. Bake until bubbly, 15 to 20 minutes.