Arrange the sweet potatoes in a shallow baking dish measuring 6x10 inches. Drizzle fresh orange juice over them and set aside.
In a separate bowl, combine flour, sugar, cinnamon, and salt. Incorporate the margarine by cutting it in with two knives or a pastry cutter until the mixture has a crumbly texture. Evenly distribute this topping over the sweet potatoes.
Bake the dish in the oven for 30 minutes.
After baking, take the casserole out and switch on the broiler. Evenly scatter marshmallows on top and broil until they become puffed and golden brown.