Preheat oven to 350 degrees F (175 degrees C). In a medium casserole dish mix together green beans, milk, cream of mushroom soup, and 1/2 of the can of onions. Bake for 25 minutes in the preheated oven, until heated through and bubbly. Sprinkle remaining onions over the top, and return to the oven for 5 minutes. Season with salt and pepper to taste.