Heat large covered saucepot salted water to boiling on high. Add green beans; cook 5 to 10 minutes or until crisp-tender. Drain well and transfer to serving dish. (Beans can be made ahead. Cool, cover, and refrigerate up to 3 days. Reheat before proceeding.).
Meanwhile, in 12-inch skillet, heat oil and margarine on medium until margarine melts. Add onion, 1/8 teaspoon salt, and pinch freshly ground black pepper. Cook 6 minutes or until golden and tender, stirring occasionally.
Stir in mushrooms and 1/4 teaspoon salt and cook 8 minutes or until tender and juices have released and begun to evaporate, stirring occasionally.\.
Add flour and cook 2 minutes, stirring, then add milk in slow, steady stream, stirring. Heat to boiling and cook 2 to 5 minutes or until thickened, stirring frequently. Stir in thyme, cayenne, and pinch salt. Spoon over green beans and top with onions if desired.
To Make 12 Servings:
Step 1: Use 3 pounds beans.
Step 2: 2 tablespoons each oil and margarine, 1/2 large onion (10- to 12-ounce), 1/4 teaspoon salt, 1/8 teaspoon pepper.