Making the tart
Preheat oven to 350F.
Skin and clean the eel, and remove the 2 lines of bones with a knife then cut eel in 3 pieces lengthwise, attached at one end of the eel. (Keep eel bones aside for onion glaze.) Braid the fillets. Pre-cook the braid in oven for about 10 minutes at 350F.
Preheat oven to 200F.
Put the onions in an ovenproof pot and fill with water to cover. Add the prunes, the garlic, and the maple syrup. Place parchment paper over the liquid, then cover with lid, and put in oven. Cook for 12 to 18 hours at 200F.
Remove onions, slice and set them aside. Strain the contents of the pot into a bowl, then put the broth back into the pot.
In a very hot saucepan, cook the eel bones in a little bit of oil for a few minutes. Then add the bones to the eel broth in other pot and reduce over low heat until the broth thickens and becomes syrupy. Add scotch and set aside.
Throw the onions, the olives, the capers, the garlic and the herring in a saucepan. Add salt and pepper. Cook for 5 to 8 minutes. Cool.
Preheat oven to 400F.
Roll the dough into a rectangle of about 45 x 30 cm (18 X 12 in.). Put on a baking sheet. Spread the onion mixture on dough and fold the sides back over the mixture about 4 cm. Glaze the dough with egg white.
Brush the partially-cooked eel braid with onion glaze, then place on top of the onion mixture. Cook for 10 minutes at 400F.
Slice the foie gras and cook in a saucepan. Remove from heat, coat with onion glaze and top the tart with the foie gras at about the 7-minute mark (that is 3 minutes before the tart is finished.) Finish cooking and serve.