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1
For the topping: Scoop preserves into a small saucepan, and place on medium low heat till melted. Stir in grapefruit juice, zest and salt. Bring to a boil, and cook till it reduces and becomes slightly syrupy and thicker, 1-2 minutes. Let cool to room temperature while you're making the pastry.
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2
For the pastry: Adjust oven rack to middle position. Heat oven to 350u00b0F. Butter or pan spray an 8- by 8- by 2-inch baking pan and line it with a piece of parchment long enough to overhang the pan's edges.
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3
Combine flour, salt, grapefruit zest, feta cheese, and butter in the bowl of a food processor. Pulse to form a dry but pebbly mixture, about twelve 1-second pulses. You can also use a pastry blender or two knives to cut the butter and cheese into the dry ingredients. Pour into the prepared baking pan, and press into an even layer.
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4
Spread liquid jam evenly over the prepared pastry. Sprinkle with chopped walnuts. Bake for 20-25 minutes. The edges will brown and come away from the sides a bit.
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5
Remove the pan from the oven and place on a rack to cool. While it is warm, press any loose nuts into the pastry, sprinkle with flaky salt (if using), and run a knife along the edge of the pan so that it's easy to remove when cool. Let it cool completely before cutting.
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6
To Serve: Carefully tug loose and lift using overhanging parchment and transfer to a cutting board. Cut into 16 two-inch squares (or any other shape you wish).
