Make soup: Toast cumin seeds in a medium-wide pot over medium-high heat until slightly darker and fragrant, about 30 seconds. Lower heat to medium and add butter. When butter melts, stir in onion. Cover and cook, stirring occasionally, until onion is translucent, about 5 minutes. Add corn, 1/2 tsp. salt, and 2 cups broth. Bring to a simmer, cover, and simmer until corn begins to soften and break down, about 15 minutes.
Working in batches, whirl corn mixture in a blender, covering lid with a towel and holding it down, until very smooth. Add cream. Chill until cold, about 2 1/2 hours, or overnight (see headnote).
Stir 2 tbsp. Champagne vinegar into soup. If soup is too thick, add a little more (chilled) broth until it's as thin as you like. Taste, and season with more vinegar and salt if you like.
Meanwhile, cook onions: Heat oil and ground cumin in a large frying pan over medium heat. Add onion slices and cook, stirring occasionally, until deep golden brown, 10 to 20 minutes. Reduce heat if they start to brown unevenly. Lift onions from oil with a slotted spoon and drain on paper towels. Reserve 1 tbsp. oil for scallops.
Cook scallops: Lay scallops on a paper towel-lined plate and bring to room temperature, about 15 minutes. Season with salt and pepper and let stand 5 minutes longer. While scallops rest, divide chilled soup among chilled bowls.
Heat a large (not nonstick) frying pan over medium-high heat. When hot, swirl in reserved onion-cumin oil, then arrange scallops in a single layer in pan with ample space between them. Cook scallops without moving them until golden and crusty on the bottom, about 2 minutes; then turn them and cook for 1 minute. Remove from heat and let scallops sit in pan 1 minute.
Set a scallop in each bowl of chilled soup. Top with a small tangle of fried onion strings, and coarsely grind pepper over all.
*Dry-packed scallops are free of brine and preservatives and brown better when you cook them.