In a blender, puree the parsley and scallions with the lemon juice, 2 tablespoons of the oil, 1/4 cup of the water, and 1/4 teaspoon each of the salt and pepper.
In a medium saucepan, bring the remaining 2 cups water to a boil with 3/4 teaspoon of the salt and 1/4 teaspoon of the pepper. Stir in 3 tablespoons of the parsley puree and the couscous. Cover, remove from the heat, and let sit for 5 minutes.
In a large nonstick frying pan, heat the remaining 2 tablespoons oil over moderately high heat.
Sprinkle the fish with the remaining 1/4 teaspoon salt and 1/4 teaspoon pepper. Dust the flounder with the flour and shake off any excess. Saut the fish until brown and just done, about 2 minutes a side for 1/2-inch-thick fillets.
Serve the fish on the couscous with the remaining parsley puree on the top.
Fish Alternatives: Any mild-tasting fish fillets will go well with the relatively delicate sauce. Try lake perch, whiting, croaker, drum, or bass, or of course, any of the flounder family, such as lemon or gray sole.
Variation: Flounder with Basil and Parsley Couscous: Make the herb puree with 1/4 cup fresh basil and 1/2 cup flat-leaf parsley leaves.
Wine Recommendation: Match the tartness of the lemon juice with an acidic white wine. Try a good-quality Soave or Orvieto from Italy.
