Preheat oven to 400°F. Warm 2 Tbsp. oil in a large, ovenproof skillet over medium-high heat. Add onion and cook, stirring, until softened, about 3 minutes. Stir in corn and cook, stirring often, until tender, about 5 minutes. Transfer skillet to oven and roast, stirring often, until vegetables are lightly browned, about 20 minutes. Immediately transfer to a plate and cool.
In a large bowl, combine corn mixture, cilantro, beans and tomatoes. In a separate bowl, mix vinegar, mustard, salt and pepper, whisking until salt is dissolved and ingredients are fully incorporated. Slowly add 1/3 cup oil, whisking constantly until blended and thickened. Pour over corn mixture. Stir well and season with additional salt and pepper, if desired.