Mix flour and salt. Grate in cold butter. Mix in cold water to make a shaggy dough, gently knead a few times. Divide in half and roll out to ¼-inch thickness. Use a wide mouth mason jar ring to cut 24 circles.
Filling:
Add all ingredients except walnuts to a large mixing bowl and whisk well.
Put pastry rounds into muffin tins. Press the pastry up the sides a bit.
Scoop filling out to divide evenly; about 2 tablespoons per pastry.
Add 1 ½ - 2 walnut halves per tart.
Bake for 15-20 minutes, rotating pans halfway through.
They will be all puffed up around the filling. Let sit 10 minutes on a cooling rack before removing from trays.
Notes
Store in an airtight container. If it’s really warm out, keep in the fridge, otherwise they live on the counter and happily get consumed within a few days!