So, I'm not going to lie, the kitchen torch makes this process significantly easier and more straightforward. But, that's not to say you don't have options. You can either serve this with the marshmallow fluff untoasted or use the grill/broiler function in your oven. You need to be cautious with this as the dessert is otherwise chilled. Cover the top of the cheesecake slice with marshmallow fluff and place under the preheated broiler, watching carefully for the marshmallow to toast. Once toasted adequately, remove for serving.