Preheat oven to 400 degrees F.
Brush fillets on both sides with oil and season with salt and pepper. Heat a large, oven-safe nonstick saute pan over high heat, add the fish, and cook until golden brown, about 2 minutes. Brush with some of the glaze, turn over, and continue cooking (on the stovetop or in the oven) for 3 to 4 minutes or until just cooked through. Brush with more of the glaze and remove from the heat.
Ladle some of the broth into large shallow bowls, place fish in the middle, glazed side up, and sprinkle some of the couscous around the fish. Serve immediately.
Place tangerine juice in a medium saucepan and cook over high heat, until reduced to 3/4 cup, stirring occasionally. While the tangerine juice is reducing, combine vinegar and sugar in a small saucepan and cook until reduced by half. Add the reduced tangerine juice and habanero to the reduced vinegar mixture and stir to combine. Puree in a blender until smooth and season with salt. Transfer to a bowl and let cool to room temperature before using.
Bring water and salt to a boil in a saucepan. Stir in the couscous, cover the pot, remove from the heat, and let sit 5 minutes or until the water is absorbed and couscous is tender. Remove the lid of the pot, fluff the couscous with a fork and stir in the zest. Keep warm.
Heat oil in a medium saucepan over medium-high heat. Add the onions and cook until soft. Add the garlic and cook for 30 seconds. Add the clam broth and cook for 15 minutes. Strain the mixture and return to the saucepan, stir in the lemon juice and bring to a simmer. Whisk in the butter, if using, and the torn parsley or cilantro. Season with salt and pepper, to taste. Keep warm.