To make the dough: Place the flour, sugar, baking powder, and salt in a large bowl. Add the eggs and vanilla, and mix together using he blades of 2 knives. Incorporate the butter bit by bit. Lightly coat your hands with a little extra flour. Using your hands, knead he dough briefly, so that it just sticks together. Be careful not to overwork the dough, or it will toughen.
This dough can also be made in a food processor, being careful not to over-process once the butter has been added.
Form the dough into 2 balls, cover each with plastic, and refrigerate at least 1 hour, and up to 12 hours. This dough can also be frozen for future use.
To make the tart: Clean the artichokes down to the "heart" and halve or quarter. Put into a 12-inch saucepan with the olive oil and onion, and season to taste with salt and pepper. Saute until onions are caramelized and then barely cover with water. Reduce heat to medium, cook for 15 minutes of until artichokes are very tender.
Drain the artichokes and onion, and transfer to a bowl. Mix in the parsley and set aside.
Prepare the bechamel sauce (see recipe for milk croquettes). Fold the artichokes into the sauce, and salt and pepper to taste.
Preheat the oven to 350 degrees F.
Grease a 9-inch springform tart pan with olive oil and lightly coat with flour.
Roll out two-thirds of the pastry into a 10-1/2-inch circle that is 1/4-inch thick. Fit the dough into the center of the pan, pressing evenly halfway up the sides.
Pour the filling into the pastry shell and smooth the top across evenly.
Take the small pieces of remaining dough and roll them into little ropes, about 1/3-inch thick. Use these ropes to stripe the top of the artichoke filling in 5 or 6 rows about 1-inch apart. Make sure to press the ends into the edge of the tart shell's dough. Turn the tart a 1/4 turn and place the remaining ropes on top of the first layer, going the opposite direction.
Bake the tart on a baking sheet in the preheated oven until golden brown, about 45 minutes, brushing crust with additional olive oil during the last 10 minutes of baking.
Remove the cooked tart and let it cool. It's best served at room temperature.
In a medium saucepan, heat butter until melted. Add flour and stir until smooth. Over medium heat, cook until light golden brown, about 6 to 7 minutes. Meanwhile, heat milk in separate pan until just about to boil. Add milk to butter mixture 1 cup at a time, whisking continuously until very smooth and bring to a boil. Cook 10 minutes and remove from heat. Season with salt and nutmeg and set aside.