This is a surprisingly easy dish considering the amazing flavours in it. The dish also provides a wonderful sauce, so make a vegetable side -- such as broccoli, cauliflower, mashed potato, etc -- to pour the sauce onto: you won't want to waste a drop of this sauce.
Use either a deep baking dish and layer things, or, a large rectangular one, and spread things out. Instead of sour cream, you may use crème fraîche. For the topping cheese, you can use any cheese that melts well such as gruyère, emmenthal, cheddar, etc. (To really zip it up, use a mild blue cheese, crumbled.) 2 oz (50g) of semi-firm cheese, grated is 1/2 cup. Instead of dried sage, you can use 3 fresh sage leaves, chopped; or another herb that you have on hand and that strikes your fancy.