Three 6-ounce bunches of watercress, tough stems discarded
1 small red onion, thinly sliced
1 mango, cut into 1/2-inch dice
1 Hass avocado, cut into 1/2-inch dice
Heat the canola oil in a small skillet. Add the ginger and cook over moderate heat until just golden and fragrant, about 2 minutes. Transfer the ginger to a small bowl and let cool. Stir in the lime juice, rice vinegar, chili paste, garlic, sugar and sesame oil. Slowly whisk in the olive oil and season with salt and pepper. Stir in the cilantro and scallion.
In a large bowl, toss the watercress and onion with the dressing and season with salt and pepper. Add the mango and avocado, toss gently and serve.
The dressing can be refrigerated for up to 2 days. Stir in the cilantro and scallion just before serving.