Cook 2 pounds green beans in boiling water for 7 minutes or until tender. Drain and place in a greased 13 x 9-inch baking dish.
In a large skillet, melt 2 tbsp margarine; cook 1 pound sliced shiitake mushrooms and 4 cloves chopped garlic for 5 minutes, stirring occasionally. Sprinkle on 1/4 cup flour, 3/4 tsp salt, 1/4 tsp pepper and 1/8 tsp dried thyme.
Gradually stir in 3 cups 2% milk and simmer 2 minutes. Pour sauce over beans; scatter 1 cup Seneca crisp onions over top. Bake at 350 degrees F for 14 minutes. Makes 8 servings.