Cook the green beans according to the package instructions, and drain well.

Preheat the broiler, and in a small pot, saute the onion in the margarine until tender, and blend in the flour, salt and pepper.

Gradually add the milk, stirring and cooking over medium heat until thickened and bubbly.

Stir in the yogurt and green beans, and heat thoroughly, about two minutes.

Transfer to 1 1/2-quart casserole, and mix with the cheese, and broil in the oven until the cheese melts, and serve.