1. Drain 1 can tomatoes, discarding juice, then purée with remaining can tomatoes (including juice) in a blender.
2. Cook onion, garlic, chile, and ginger in oil in a 4- to 5-quart heavy nonreactive pot over moderate heat, stirring frequently, until onion is softened, about 8 minutes.
3. Add cumin and cook, stirring, 1 minute. Stir in puréed tomatoes, broth, 1 tablespoon sugar, and 2 teaspoons salt and simmer, uncovered, stirring occasionally, 20 minutes.
4. Working in 3 or 4 batches, blend soup in blender until smooth (use caution when blending hot liquids). Transfer soup as blended to a sieve set over a large bowl and force through sieve, discarding seeds.
5. Stir in sugar and salt to taste. Reheat in cleaned saucepan if necessary.

Serves 4.