1. Fill a large bowl with water and add plenty of ice. Set aside. Fill a large pot halfway with water. Add the 1/4 cup dill, the garlic, a nice pinch of salt, and a few grinds of black pepper. Bring to a boil and simmer until the pot is fragrant, about 5 minutes.
2. Add the prawns and remove the pot from the heat. Stir, allowing the prawns to cook until pink and opaque, 2 to 3 minutes. Drain the pot in a colander and pour the prawns into the ice bath to cool.
3. To make the salad, in a large bowl, combine the dill, grated garlic, mayonnaise, sour cream, yogurt, cucumber, onion, lemon zest, and sugar. Taste, and then season with a pinch of salt and a few grinds of black pepper. Remove the prawns from the ice bath, dry gently with a paper towel, and coarsely chop. Stir them into the dressing until combined.
4. Tear the first few layers from the center of each iceberg wedge (creating a "bowl" in the wedge) and put them on individual salad plates. Divide the prawn salad among each plate, filling the lettuce "bowl" and letting the salad spill over onto the wedge.
5. Garnish with chopped dill. Serve cold or at room temperature.