Preheat the oven to 375 degrees F (190 degrees C). Lightly grease a casserole dish.
Stir the milk, sour cream and cream of mushroom soup together in a large bowl. Fold in the green beans and spinach, and mix in about half of the onions. Pour into the casserole dish and top with the remaining onions.
Bake uncovered in the preheated oven until bubbly and browned on top, about 40 minutes.