There is a particular kind of cold that settles over a Canadian kitchen in January — the kind where the windows frost at the edges and dinner needs to be the sort of thing you can smell from the hallway. This slow cooker vegetarian shepherds pie was built for exactly that. A deep, savoury lentil-and-vegetable filling simmers all afternoon while you get on with your day, and it gets crowned with a fluffy sweet potato topping that goes just a little caramelized under the broiler. Make it once on a Sunday and you have eight proper dinners waiting in the freezer, each one costing less than a coffee-shop muffin. Key…