Called a supergrain, quinoa dates back to the Incan civilization. It has more protein than most grains (5 grams per serving) and is gluten- and wheat-free.
Whisk olive oil, vinegar, basil, mustard, and garlic in a large bowl to blend. Season to taste with salt and pepper.
Add the quinoa, corn, and peppers to the bowl; lightly toss to combine.
Arrange watercress on a large platter. Mound the salad in center of platter. Garnish with scallions.
If you prefer, you can replace the watercress with fresh baby spinach leaves.