I completely agree that an oven can dry out some dishes and it is a common problem with food made in advance. If cooked in one go, going straight from the hob to under the grill to finish off is a good strategy. Blow torches have their place but by and large I prefer a grill to finish off this kind of dish. I don't only want colour, I want to achieve a crunchy, crispy texture on a gratin/cheese topped dish.