Working over a large bowl, squeeze the yellow tomato halves to release the seeds and juices. Reserve the liquid.
Combine the squeezed yellow tomatoes and salt in a food processor or blender and puree until smooth. Transfer to the bowl containing the tomato pulp and juice. Add the onion, garlic, and lemon juice to the food processor or blender and puree until smooth. Strain the onion liquid into the bowl containing the tomatoes. Add the jalapeño and mix well. Refrigerate until ready to serve.
Serve the gazpacho chilled, garnished with the cilantro, cucumber, and diced tomato.