strawberry rhubarb cobbler with gingered biscuit topping

adapted from Sara Moulton

cobbler:
4-5 pints strawberries, hulled, rinsed and quartered
1-1.5 lbs rhubarb, peeled and cut into .5-1" wide pieces
1/3 cup sugar, or to taste
2 T all-purpose flour
2 t finely grated lemon zest
generous pinch of salt
1 T unsalted butter, cut into slivers

topping:
1 c all-purpose flour
3 T sugar
1 t baking powder
1/4 t baking soda
1/4 t salt
4 T very finely chopped crystallized ginger
4 T unsalted butter, cut into small pieces
1/2 c buttermilk, plus more for brushing

Put rhubarb in a large, heavy saucepan. Add a tablespoon or two of the sugar and stir over high heat until the sugar has started to dissolve, 1-2 minutes. Reduce the heat to medium and cook, stirring often, until the fruit has exuded some of its liquid, about 5 minutes.

In a large bowl, combine the strawberries and sugar, then add the rhubarb mixture (if the rhubarb has given off a lot of liquid, you may not want to add all the liquid here), lemon zest, the rest of the sugar, and the salt. Let cool a bit, then pour into a lightly buttered baking dish, and dot with the slivers of butter.

Preheat oven to 400 degrees F. Combine the flour, sugar, baking powder, baking soda, and salt in a large bowl. use a pastry blender or your fingertips to mix in the ginger and to cut in the butter until the mixture resembles the texture of small peas. Quickly stir in the buttermilk to form a soft dough. Turn out onto a well-floured work surface, knead into a ball, and roll out to a thickness of about .5". Cut out biscuits using a cookie cutter or thin rimmed glass about 2" in diameter.

Arrange the biscuits tightly over the fruit. Brush lightly with additional buttermilk, sprinkle with demarera sugar, and bake until cobbler is bubbly and biscuits are golden brown, 35-45 minutes. Serve hot, possibly with vanilla ice cream.