1/3cupcrème fraîche (or sour cream), plus extra for serving, (2 1/2 ounces)
1cupchopped nectarines, from above
Make the topping:
Position a rack in the center of the oven and preheat to 350ºF.
Place the butter in a 10" oven-proof skillet with 2" high sides, and put it in the oven to melt the butter. Brush some of the butter up the sides of the pan. Sprinkle the brown sugar over the butter, and place in the oven until the brown sugar is moistened and bubbling, 5 minutes.
Halve and pit the nectarines, and slice them fairly thin, about 1/8" thick. Set aside the end pieces. Beginning with the largest pieces, place the nectarine slices over the brown sugar with the curved edge facing out, overlapping the fruit as you go. Continue to make circles with the nectarine slices until you've covered the pan. Chop the remaining slices and end pieces; you should have 1 cup. Set these aside to add to the batter.
Make the cake batter:
In the bowl of a stand mixer fitted with the paddle attachment, beat together the butter, sugar, vanilla seeds, and rosemary on medium speed until light and fluffy, 3 minutes, scraping down the sides of the bowl occasionally. Beat in the vanilla extract, then the eggs one at a time. (The mixture may look curdled; this is O.K.)
Meanwhile, sift together the flours, tapioca starch, xanthan gum, baking powder, and salt into a medium bowl.
With the mixer on low speed, stir in half of the flour mixture, beating until just combined. Beat in the crème fraîche until combined, then the remaining flour mixture. Scrape down the sides of the bowl, and give the batter a stir to make sure it is mixed thoroughly. Fold in the chopped nectarines by hand until just combined.
Dollop the batter over the nectarines in the pan, and spread it gently and evenly.
Bake the cake until a tester inserted in the center of the cake comes out completely clean, about 45 minutes. Let the cake cool for 5-10 minutes (but no more, or the caramel may harden, causing the fruit to stick to the pan. If this happens, put the cake back in the oven for 5 minutes to re-melt the caramel).
Invert a large plate or platter over the pan, grasp it tightly with two oven-mitted hands, and bravely flip the whole thing over. The cake should release easily from the pan.
Let the cake cool to warm, 20 minutes, then serve warm or at room temperature, dolloped with crème fraîche. The cake is at its most gooey and pie-like on the first day, but leftovers will keep at room temperature for an additional day, or in the fridge for a few. Re-warm before serving for best results.
I think this cake would be awesome made with ripe pears in place of nectarines; apricots, plums, or, of course peaches could all stand in for the nectarines, too.If gluten isn't an issue, use 1 1/4 cups all-purpose flour in place of the rice, oat and millet flours and omit the tapioca starch and xanthan gum. If you or your guests are highly sensitive to gluten, be sure to seek out flours and other ingredients that are certified gluten-free. Also, be sure to use sweet rice flour here (such as Mochiko brand); this is made from sticky rice, and has a finer, stickier texture than regular white or brown rice flours.You can make your own crème fraîche by stirring 1 tablespoon buttermilk into 1 cup heavy cream and letting it sit, covered, at room temperature for 24 hours. Stir it a few times as it sits, then cover and refrigerate. It should keep for at least 1 week.All ounce measurements here are by weight.Nutritional values are based on one of ten servings.