Prepare the Crust:

1. In the bowl of a food processor, combine flour, ground almonds, sugar, salt, and orange zest, and pulse briefly to mix. Add the cold butter, and pulse five or six times, until mixture resembles coarse meal. With the machine running, add egg yolk, then add the ice water, one tablespoon at a time, stopping as soon as the dough starts to clump together.
2. Turn the dough out onto a clean surface and gently form it into a flat disk. Wrap in plastic, and refrigerate for at least one hour or until quite firm.
3. On a lightly floured surface, roll out the dough into a 12" circle. Gently fit into a 9" tart pan, folding in the overhang and pressing against the side to create a thicker rim. Freeze until firm, about 10 minutes.
4. Meanwhile, preheat oven to 375F.
5. Line the tart shell with a sheet of aluminum foil or parchment paper and fill with pie weights or dried beans. Bake for 20 minutes, then carefully remove the foil and weights, and return to the oven to bake for another 10 minutes or until golden brown all over.
6. Reduce the oven temperature to 350F, and set the tart crust aside to cool on a wire rack while you begin on the curd.

Prepare the Filling:

1. In a heavy-bottomed saucepan set over medium-high heat, whisk together the orange zest, blood orange juice, lemon juice, sugar, eggs and egg yolk. Cook the curd, stirring constantly with a heat-proof spatula, for about 5-10 minutes or until it starts to thicken and coats the back of the spatula. (If the curd should happen to curdle at any point, run it through a fine mesh strainer to remove the curdled bits - or, if it's really badly curdled, simply buzz it with an immersion blender until smooth. No one will ever know!)
2. Remove from heat and stir in the butter pieces until melted. Set aside to cool for 10 minutes.

Bake the Tart:

1. Place the prepared shell on a baking sheet. Pour the cooled curd into the shell, smoothing it out into an even layer.
2. Bake in preheated oven for 25-30 minutes, or until the filling is lightly puffed and set on the edges, but still retains a slight wobble in the middle. Let cool to room temperature on a wire rack, then transfer to the refrigerator to chill for 2-3 hours before serving.