Swiss chard and sweet potato gratin

I was really excited about making this because I thought the sweetness of the sweet potatoes would combine perfectly with slightly bitter greens. But something about this recipe just isn't right, either the proportions, the assembly, or the timing, or maybe all 3.

I ended up cooking my gratin for 80 mins, 20 mins more than Deb calls for, but the sweet potato was STILL undercooked and hadn't absorbed enough of the roux sauce. So the end result was an unpleasant combination of chewy, undercooked potato, with slightly-too-watery sauce.

I still have faith in the idea of a sweet potato and chard gratin, but I just haven't gotten around to taking the time to fiddle and make this just right. Does anyone who's made this have any tips? Did yours turn out similarly? Or wonderfully?

Things that will need tweaking: consistency of the roux; cooking time; something about the sweet potatoes (parboiling?? bake the whole thing for twice as long?? I don't know).