For the brine: In a medium saucepan, bring 1 quart water to a boil. Add the salt, ginger, peppercorns and star anise, if using. Turn the heat to low and simmer 10 minutes. Pour into a large container with 1 quart orange juice and 2 quarts water and stir well. Add the turkey legs and refrigerate 8 hours or overnight (no more than 18 hours).
Preheat the oven to 400 degrees F. Line a baking sheet with foil. Remove the turkey legs from the brine and place on the prepared baking sheet. Mix together the remaining cup orange juice and 1 cup water and pour into the pan. Cover tightly with aluminum foil, and bake for 40 minutes.
For the glaze: Meanwhile, in a small saucepan over medium heat, cook the brown sugar with 1 cup water, stirring occasionally with a wooden spoon, until the sugar begins to melt. Reduce the heat to low and continue to cook until the caramel begins to bubble, about 4 minutes.
Remove the pan from the heat and carefully pour in the soy sauce 1/4 cup at a time (the caramel will bubble and steam, so keep it at an arm's distance and allow it to calm down between additions). Stir in the fish sauce, scallion whites, garlic and jalapenos. Return to medium heat and cook, stirring occasionally, until the sauce comes to a simmer. Remove the foil from the turkey legs. Baste the turkey legs with half of the caramel sauce, flipping the legs over to coat both sides, and bake for another 20 minutes. Repeat the process with the remainder of the sauce, and continue to cook until an instant-read thermometer inserted into the thickest part of the leg registers 165 degrees F, about 15 more minutes. Remove to a serving dish and cover with foil, then allow to rest for at least 15 minutes before serving. Garnish with the scallion greens.