In a small glass bowl, combine the salt, pepper, and vinegar and stir until the salt is dissolved. Whisking constantly, drizzle in the olive oil and continue whisking until the dressing is emulsified. Still whisking, add the basil, cilantro, and garlic. Cover with plastic wrap and allow to sit at room temperature for 1 hour so that the flavors can marry.
In a large serving bowl, combine the watercress, tomatoes, and onion rings and toss gently to mix. Drizzle on the dressing and toss again just until the ingredients are evenly coated.