Blanch the haricots verts in hot water for about 3 to 5 minutes then remove from heat and shock in iced water. Drain and set aside to cool.

Preheat oven to 325 degrees F.

In a large skillet saute the chopped onion in butter. Then add the cooled beans and saute for a couple of minutes. Add the canned mushroom soup and mix well.

Transfer to a buttered casserole dish and top with the diced tomato, French-fried onions, and season with salt and pepper, to taste. Bake in oven for about 10 minutes.