Trim the tough membrane on the outside of the steak, leaving some of the fat, as it provides flavor. Then cut into 1½" strips.
In a large bowl, combine the salt, 3 cups lemongrass sauce, Thai chili and sodium nitrate and mix well. Add the flank steak strips, toss and let marinate in the refrigerator for 48 hours. After one day, toss the steak strips, then continue to marinate for a second day.
Place a wire rack into a sheet pan. Remove the meat from the marinade and place it onto the wire rack. Strain the marinade and place the strained ingredients evenly onto the meat. Bake at 250°F for 1¼ to 1½ hours until the internal temperature reaches 150°F.
Remove the pipikaula from the oven; cut about 1 lb. of the pipikaula into ¼" thick pieces.
In a large pan over medium heat, add the oil, onions and chili flakes, if using and stir to coat. Sauté the onions for 3 to 4 minutes until slightly brown. Add the sliced pipikaula and ¼ cup lemongrass sauce, to taste, and combine well. Continue cooking until heated through, about 1 minute. Add the green onion, stir and serve. Enjoy with an ice cold beer.