Chop the onion.

Melt butter in a large saucepan over medium heat.

Sauté the chopped onion in the melted butter until tender.

Stir in flour, salt, pepper, and ground mustard until well blended.

Gradually add milk, stirring continuously to avoid lumps.

Bring the mixture to a boil, then reduce heat to low.

Cook and stir for 2 minutes, or until the sauce has thickened and is bubbly.

Reduce heat to low and stir in the shredded Swiss cheese until melted and smooth.

Stir in the thawed and drained green beans until they are evenly coated in the cheese sauce.

Transfer the mixture to a greased 2-quart baking dish.

Sprinkle the toasted slivered almonds evenly over the top of the casserole.

Bake, uncovered, in a preheated oven at 350°F (175°C) for 25-30 minutes, or until bubbly and lightly browned.