Immerse tomatoes in boiling water to cover for 5 seconds (if ripe) to 15 seconds (if firm). Lift out and drain. Core tomatoes and pull off skin; discard cores and skins. Halve tomatoes crosswise; squeeze out juice and seeds and discard.
In a food processor, combine seeded tomatoes, onion, garlic, serrano chiles, and red wine vinegar. Pulse until finely chopped. Pour into a bowl.
In processor, pulse cucumber until coarsely chopped. Add to tomato mixture and season with salt to taste.
Note: Nutritional analysis is per tbsp.