1. In a blender or a mini food processor, combine the chives, parsley, oil, garlic, ginger, salt, fish sauce, black pepper, and red pepper flakes. Process until everything is finely chopped and well mixed. If you are using a blender, you may need to add a little more oil to make everything blend smoothly.
2. Season the lamb all over with salt and pepper. Then, using the tip of a paring knife, make vertical slits between the bones of the racks of lamb (the slits should be just large enough for a finger to fit in, so you can push in the marinade). Rub the herb mixture all over the lamb, including pushing it down into the slits. Let the lamb marinate at room temperature while you prepare the potatoes.
3. Heat the oven to 475°F.
4. Put the potatoes in a pot, add water to cover, and season the water with salt until it tastes salty. Bring the water to a boil. Cook until the potatoes are tender when poked with the tip of a knife, 10 to 15 minutes, depending on their size. Drain them well, pat them dry, and place them in a large roasting pan or a rimmed baking sheet.
5. Using your hands or the back of a spoon, flatten the potatoes until they crack open. Drizzle them with olive oil and sprinkle with salt.
6. Place the racks of lamb on top of the potatoes, fat-side facing up (the bones should curve downward). Place the pan in the oven and roast for 5 minutes. Then reduce the oven temperature to 425°F and continue to roast until the lamb is done to taste, another 18 to 23 minutes for rare meat (115°F to 120°F on a meat thermometer). Or roast the meat for a bit longer if you like it mediumrare or medium.
7. Transfer the lamb to a cutting board and tent it with foil to keep warm (the internal temperature will also rise by a few degrees as it sits). Meanwhile, raise the oven temperature to 500°F, flip the potatoes over, drizzle a little more oil over them if they look dry, and continue to roast until they are golden, 8 to 12 minutes.
8. Cut the lamb into chops, slicing between the bones, and serve them sprinkled with chive blossoms, if you’ve got them, pulled apart into individual florets, along with the potatoes and with mustard on the side.
Lamb: You can marinate the lamb for up to 2 hours at room temperature, or covered and refrigerated overnight.
Potatoes: You can boil and store the potatoes, loosely covered, in the refrigerator for up to 5 days in advance. Crush the potatoes right before roasting them.6 to