At least an hour or up to three days in advance, cover the sliced red onions with the red wine vinegar and a pinch of salt, until they become softened and turn a brighter pink. Keep refrigerated. (If you prefer a punchier pickled onion, heat the vinegar before pouring over the onions.)
Grate the garlic clove into a jar. Add the olive oil, and let sit for a few minutes to allow the garlic to mellow.
Add the remaining ingredients, and season to taste with salt and pepper. Shake to mix well. Extra dressing will last in the fridge for up to a week.
Grill the corn over medium heat, turning frequently, until evenly grilled, 3-5 minutes. Remove from heat. Once cool, cut the corn from the cob, and set aside.
In a large bowl, toss the lettuce with the dressing. Starting on one side of the bowl, make a row of the cherry tomatoes, followed by the remaining ingredients, each in their own row. Drizzle more dressing on top.