This is an incredible dish – though it’s not that obvious from reading the original recipe. I also suspect the kind of sweet potatoes is important – i used white jersey – they’re not as sweet.
Basically this gratin is an exciting exercise in both texture and flavor: soft potato flesh vs crunchiness of onions, potato sweetness vs heat of chiles. I tweaked the recipe a bit – below is my version.
Cut about 2.5 lb of peeled sweet potatoes into 1/3-1/2" slices, arrange with a slight overlap on a large skillet, drizzle with couple tbs of melted butter, salt and roast for about 35mins on 425F. In the meantime char 4 poblanos, peel, tear into 1/2/" strips; halve 1 white onion, and cut into 1/4" half moons; mix onion with poblano and mix 1/4 cup of creme fraiche. Top potatoes with onion/chile mixture, lower the oven temp to 375F and roast for about 35mins more. The bottoms of potatoes will be nicely caramelized at this point.
For the side i made a small prosciutto/parsley salad: it’s amazing – i made and posted about it before.