Preheat the oven to 350 degrees.
Blanch the haricot vert until they are al dente, about 3 minutes.
Melt 3 T butter in a dutch oven. Add flour. Stir and cook until the flour is a light brown color. Stir in the salt, onion, sour cream and green beans. Transfer the mixture to a casserole dish.
Spread cheese over top. Add 1 T melted butter to bread crumbs and parmesan cheese, mixing thoroughly. Sprinkle over casserole. Bake for 30 minutes until golden brown and bubbly.