Roast the peppers under the broiler until the skins are dark brown. Turn them frequently to brown and blister the surface all around. Remove the peppers to a covered bowl and let them steam for a few minutes.
When the peppers are cool enough to handle, pull the stems out, cut the peppers open, and remove the seeds and membrane from the inside. Peel the brown skin off. Do all this over the bowl and save the juices.
Slice the peppers into medium dice and toss with the collected juice in a large bowl.
Bring a large pot of water to a rolling boil. Shuck the corn and put it into the boiling water. When the water returns to a boil, turn the heat off and let the corn steep in the water for five minutes more.
When cool enough to handle, cut to corn off the cob with a sharp knife. Add to the bowl with the roasted peppers.
Whisk the garlic, vinegars, Tabasco, and salt to make a vinaigrette. Pour it over the corn and pepper salad.
Cover and refrigerate long enough to cool. Add the watercress leaves, green and red onions, and toss.