Nutty buckwheat pairs perfectly with the tartly sweet berries, balanced by a bright kick of ginger.
9 tablespoons unsalted butter, divided, plus more for greasing pan
½ cup buckwheat flour
½ cup plus 2 tablespoons gluten-free flour blend (or all-purpose flour)
1/3 cup plus ½ cup granulated sugar, divided
¾ teaspoon salt, divided
2–3 tablespoons cold milk or water
3 cups fresh elderberries
1/3 cup light brown sugar
2 teaspoons fresh grated ginger
2–3 tablespoons cornstarch
1 tablespoon lemon juice
Lightly grease the bottom and sides of an 8-inch or 9-inch tart pan. Set aside.
Mix flours, 1/3 cup sugar, ½ teaspoon salt, and cut in 7 tablespoons butter. With pastry blender, fork, or hands, mix butter into flour mixture until the dough resembles small peas. Sprinkle in milk or water by the teaspoon and blend until dough sticks together when pressed, but is still crumbly.
Press dough into and up sides of pan, chill for 30 minutes. Preheat oven to 350° and bake for 15 minutes until golden-brown. Remove from oven and allow to cool.
While tart shell is cooling, add elderberries, remaining ½ cup granulated sugar, brown sugar, remaining ¼ teaspoon salt, and ginger to a large pot and heat on medium-high. Blend cornstarch with 1 tablespoon water to create a slurry, and then add to the elderberry mixture, stirring to dissolve slurry and sugars.
Bring to simmer and continue cooking, stirring frequently, until mixture thickens, about 5 minutes. Add another tablespoon of water-thinned cornstarch if needed to get mixture to thicken.
Turn off heat, stir in lemon juice, and pour elderberry mixture into cooled tart shell. Dot with remaining 2 tablespoons butter and bake for 40 minutes until set.
Serve warm or at room temperature, ideally with vanilla ice cream.