Add olive oil, lemon zest and a few grinds of black pepper to a large saute pan over medium-low heat. Cook until oil is warm and pepper is fragrant when you waft it towards you, about 4 minutes.
Add the onions, garlic, scallions, a pinch of chile flakes and a pinch of salt and cook, stirring occasionally to prevent them from browning, until the green of the scallions are wilted and darkened and the onion and garlic are tender, about 4 minutes. Add the tomatoes and cook, stirring, until softened, about 2 minutes. Add clam juice and lemon juice and bring to a light simmer; stir. Taste: If the tart of the tomatoes and lemon juice is too strong, add sugar in sprinkles, stirring and tasting as you go until it tastes just right. You may not use it at all or you may not use it all; it's totally optional.
Raise the heat to medium and nestle the shrimp into the pan. Cover and cook on one side until shrimp begins to turn opaque and curl slightly, 2 to 3 minutes. Toss and cook until shrimp just begins to turn pink on the other side, 1 to 2 more minutes. (Shrimp cooks fast and there's a thin line between done and overdone; the best rule is to remove the pan from the heat as it begins to reach the finish line.) Continue tossing and the heat of the pan will continue to cook the shrimp until done. Add pasta to a large serving bowl, then sprinkle with parsley, drizzle with olive oil, sprinkle with some more salt and pepper and top with the shrimp mixture. Serve warm during the winter or chilled during the summer.