2 cups water
1 cup sugar
2 blood oranges, thinly sliced
1 cup shelled pistachios
½ cup unsalted butter, cut into pieces
½ cup sugar
1 teaspoon grated orange rind
2 eggs, beaten
1 pre-baked 9-inch tart shell
1 tablespoon coarsely chopped pistachios
Place water and sugar in a wide pot over high heat and bring to a boil, stirring to dissolve sugar. Reduce heat to medium-low, add orange slices and simmer, turning oranges occasionally, for 40 to 45 minutes or until oranges are soft and peel is beginning to look translucent.
Transfer slices to a rack to cool, reserving syrup for glaze. If syrup is too thin, boil down until it thickens.
Preheat oven to 375F.
Combine pistachios and butter in a food processor. Sprinkle in sugar and orange rind and pulse until mixture is combined. Add salt and eggs and process until incorporated.
Spread mixture in pre-baked tart shell and bake 25 to 30 minutes (covering the edge of the pastry with foil if it begins to get too browned), or until filling is puffed and browned. Cool.
Fan orange slices decoratively over top of tart and brush with orange glaze. Sprinkle with chopped pistachios.