1 1/2 cup gluten-free flour
2 tablespoons sugar, plus 1 teaspoon for sprinkling
¼ teaspoon salt
4 tablespoons cold butter, ( ½ stick), cut into small pieces
1 tablespoon canola oil
¼ cup ice water, plus more as needed
1 large egg, separated
1 teaspoon lemon juice, or white vinegar
Filling & glaze
¼ cup slivered almonds, (1 ounce)
¼ cup gluten-free flour
¼ cup plus 3 tablespoons sugar
4 cups mixed berries, such as blackberries, raspberries and blueberries
2 teaspoons lemon juice
1 tablespoon water
To prepare crust: Whisk flour, 2 tablespoons sugar and salt in a medium bowl. Cut in butter with a pastry blender or your fingers until the mixture resembles coarse crumbs with a few larger pieces. Add oil and stir with a fork to blend. Mix ¼ cup water, egg yolk and 1 teaspoon lemon juice (or vinegar) in a measuring cup. Add just enough of the egg yolk mixture to the flour mixture, stirring with a fork, until the dough clumps together. (Add a little water if the dough seems too dry.) Turn the dough out onto a lightly floured surface and knead several times. Form the dough into a ball, then flatten into a disk. Wrap in plastic wrap and refrigerate for at least 1 hour.
Preheat oven to 425°F. Line a baking sheet with parchment paper or foil and coat with cooking spray.
To prepare filling & assemble tart: Spread almonds in a small baking pan. Bake until light golden and fragrant, about 5 minutes. Let cool. Combine , ¼ cup sugar (or Splenda) and the toasted almonds in a food processor or blender; process until the almonds are ground.
Roll the dough into a rough 13- to 14-inch circle on a lightly floured surface, about ¼ inch thick. Roll it back over the rolling pin, brush off excess flour, and transfer to the prepared baking sheet. Spread the almond mixture over the pastry, leaving a 2-inch border all around. Toss berries with the remaining 3 tablespoons sugar and 2 teaspoons lemon juice in a large bowl; spoon over the almond mixture. Fold the border up and over the filling, pleating as necessary. Blend the reserved egg white and 1 tablespoon water with a fork; brush lightly over the tart rim. Sprinkle with the remaining 1 teaspoon sugar.
Bake the tart for 15 minutes. Reduce oven temperature to 350 degrees and bake until the crust is golden and the juices are bubbling, 30 to 40 minutes. Leaving the tart on the parchment (or foil), carefully slide it onto a wire rack. Let cool.
Shortly before serving, melt jam in a small saucepan over low heat; brush over the berries. Cut the tart into wedges.