Hollowed grape tomatoes become diminutive salad bowls, holding a few leaves of lightly dressed greens called tatsoi.
Process vinegar, shallot, garlic, mustard, basil, chives, salt, and pepper in food processor until smooth. With machine running, drizzle in oil until emulsified. Transfer to squeeze bottle with small tip.
Cut sliver off bottom of each tomato and 1/8 inch off top. Using a mini melon-baller, scoop out centers. Squeeze a small amount of dressing into each tomato; top with greens. Serve immediately.