Sometimes a dollop of «sweet cream» works just perfect (this is a mix of greek yogurt and / or whipped cottage cheese with a tsp or two of truvia / stevia, maybe a few drops of vanilla as well).
Elana's Pantry posted this recipe for Paleo Shepherd's Pie recently using cauliflower mash for the topping and a few months back Jan's Sushi Bar posted this Sweet and Spicy Cottage Pie using mashed vanillasweet potatoes on top.