How to Make It
Preheat the oven to 350°. In a large bowl, stir the kosher salt and tarragon into the water. The mixture should be slushy; if you squeeze a handful, it should just hold together. At either end of a large roasting pan, using half of the salt mixture, form 2 ovals slightly larger than the lamb racks.
In a large skillet, heat the 3 tablespoons of olive oil. Add the lamb, meaty side down, and cook over moderately high heat until browned, about 4 minutes. Turn the racks and brown the tops and sides for 2 minutes each. Set the racks on the beds of salt and let the lamb cool slightly.
Pat the remaining salt mixture over and around the lamb racks to cover the meat completely. Roast for 25 minutes, until an instant- read thermometer inserted in the center of the meat registers 135°. Remove from the oven and let rest for 5 minutes.
Remove the salt crust and brush any remaining salt off the meat. Brush the lamb with olive oil, then cut into single chops for serving. Sprinkle with flaky sea salt.