Preheat oven to 375 degrees F. Mix flour, sugar, baking soda, baking powder, salt, butter, buttermilk, and egg. Knead lightly and divide into 4 portions, forming each into a disk. Place the 4 disks on a lightly greased baking sheet and bake until a toothpick inserted in center comes out clean, about 30 minutes. Let cool on pan for 10 minutes, then cool completely on a rack.;
Heat grapeseed oil over low heat in a medium skillet. Add onion and curry powder, and allow onion to slowly caramelize, about 30 minutes. Stir in golden raisins and add vinegar. Cook for about 5 minutes, cover, remove from heat and allow to rest.
Return to the Crostini: Split loaves open as you would a hamburger roll. Using a 1-inch circle cutter cut the bread into rounds.
Preheat oven to 375 degrees F. Heat oil in a large skillet over medium-high heat.
Season lamb with salt and pepper and sear lamb on all sides. Combine chestnut puree and stone ground mustard in a small bowl and coat lamb with the mixture. Then coat with the ground oats. Place in baking pan, and finish in the oven to an internal temperature of 140 degrees F, as measured with an instant-read thermometer, or to your liking. Let rest for 5 minutes then slice into 1/4 inch thick disks.
Toast crostini rounds, top with a slice of lamb loin and top with onion marmalade.