Saute 1 finely chopped sweet onion in 2 tablespoons vegetable oil in a medium skillet over medium-high heat, until browned and very tender, 20 to 25 minutes; let cool. Puree 1 finely chopped medium zucchini in a food processor with 1/2 cup mayonnaise, 1/4 cup each chopped dill and scallions, 2 teaspoons white wine vinegar and 1/2 teaspoon each onion powder, granulated garlic and kosher salt. Stir in 1 1/2 cups light sour cream; season with salt. Stir in the sautéed onion. Top with more dill.