Halve the nectarines along their natural seam, but leave skins on. Cut each half into 3 wedges. Toss nectarines with blueberries, sugar, and tapioca and put into a buttered 23-cm round gratin or casserole dish. Dot the top of the fruit with the pieces of butter.
For the cobbler top:
Whisk the flour, sugar, baking powder, and salt in a medium bowl. Rub in 1/3 of the butter with your fingertips until no visible pieces remain. Rub in the remaining butter just until it is in even, pea-size pieces. Whisk together the egg and cream and stir into the dry ingredients to make a shaggy, loose dough.
To assemble the cobbler:
1) Position a rack in the middle of the oven and preheat to 190C/Gas 5.
2) Spoon large spoonfuls of dough on top of the fruit in clumps (it should look like rough, old-fashioned cobblestones, hence the name cobbler).
3) Bake until golden brown and a toothpick inserted in the center of the topping comes out clean, about 1 hour. Cool cobbler on a rack, about 20 minutes.
4) Serve warm or room temperature, with vanilla ice cream or whipped cream, if desired.
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