Heat the oil in a large soup pot or dutch oven over medium heat; add the onion and cook, stirring, until tender, 5 minutes.
Stir in the ginger and cook 1 minute more.
Add the tomato and salt; bring to just a boil.
Reduce heat and simmer, stirring occasionally, until the tomatoes are soft and juicy, about 15 minutes.
Pass the tomato mixture through a food mill or sieve (to remove the seeds and skin) and return it to the pot.
Allow to cool to room temperature.
Stir in lemon juice and enough cold water to make 6 cups.
If serving immediately, divide among 6 soup bowls and top each with 1 or 2 lemon slices.
Or cover and refrigerate up to 2 days; let stand 30 minutes at room temp before garnishing and serving.
1 point per serving (1 cup).