Cook broccoli in boiling water 5 minutes or until tender; remove broccoli with a slotted spoon, and drain. Return water to a boil; add spaghetti. Cook pasta according to package directions, omitting salt and fat. Drain; set aside.
Preheat oven to 450°.
Melt the butter in a large nonstick skillet over medium-high heat. Add the mushrooms, oregano, basil, and garlic; saute 4 minutes. Lightly spoon flour into a dry measuring cup; level with a knife. Stir flour into mushroom mixture. Gradually add Chicken Stock and milk; stir well with a whisk. Bring to a boil, and cook 5 minutes or until thick, stirring occasionally. Add cheese, sherry, salt, and pepper; stir well. Remove from heat, and stir in chicken.
Arrange 3 cups spaghetti in a 13 x 9-inch baking dish coated with cooking spray. Top with 2 cups broccoli and half of chicken mixture. Repeat layers. Sprinkle with breadcrumbs. Bake at 450° for 15 minutes or until golden brown.