For this dish, try to use as colourful a selection of ripe, sweet tomatoes as possible. The so-called heirloom varieties can be good (although very expensive), but if they're not in season, cherry tomatoes, and the oval and pear-shaped small tomatoes, are usually better than regular large supermarket varieties.
If you're serving fewer people, use half the amount of tomatoes (or less), but make the full recipe for the dressing. Store the leftover dressing in a container in the fridge; it keeps for several weeks, and it takes only a few minutes to slice more tomatoes the next time you want this salad.
If you don't have a food processor, blender or immersion blender, the dressing can be made with a bowl and whisk
Roughly chop the onion then put it in the bowl of a food processor or blender (or use an immersion blender). Add the sugar, vinegar and soy sauce and process to a smooth purée. Pour the cooking oil and sesame oil into a liquid measuring cup with a pouring spout. With the motor running, add the oils in a slow, steady stream through the feed tube. Scrape the mixture into a bowl or jar. (If you don't have a food processor or blender, chop the onion to a paste and put it in a bowl with the sugar, vinegar and soy sauce, then whisk to combine. Pour the cooking oil and sesame oil into a liquid measuring cup with a pouring sprout. Whisking constantly, add the oils in a slow, steady stream, to create an emulsion.)
Rinse the tomatoes and dry them with a clean dish towel. With small, bite-sized tomatoes, cut them in half. For larger tomatoes, cut some in wedges and others into slices about 6mm ( ¼in) thick. Put all the tomatoes in a mixing bowl and drizzle with some of the dressing. Use your hands to gently combine the ingredients. Arrange the tomatoes on a plate and add the leaves and greens. Sprinkle with the toasted sesame seeds and serve, with more dressing on the side.