In a large saucepan, saute onion in butter until tender. Stir in flour until blended. Gradually add broth and milk. Bring to a boil; cook and stir for 2 minutes or until thickened.
Remove from the heat; stir in cheese.
Pour 1/3 cup each into two greased 8-in. square baking dishes; set aside.
In a large bowl, combine 1 cup cheese sauce, chicken, broccoli, eggs, bread crumbs, pimientos, parsley and salt if desired.
Spread about 1/2 cup over each noodle.
Roll up jelly-roll style, beginning with a short side; secure ends with toothpicks.
Place six roll-ups curly end down in each baking dish. Top with remaining cheese sauce.
Cover and freeze one casserole for up to 3 months. Cover and bake second casserole at 350° for 40 minutes or until a thermometer reads 160°. Uncover; bake 5 minutes longer or until bubbly. Discard toothpicks.
To use frozen casserole: Thaw in the refrigerator for 8 hours or overnight.