Heat grill to high (about 500°F). Combine first 6 ingredients in a bowl; stir with a whisk. Set aside.
Toss long beans with 1 tablespoon canola oil in a large bowl. Brush onion rings with remaining 1 tablespoon oil. Grill, turning occasionally, until vegetables are tender and charred in spots, about 8 minutes.
Cut beans in half, and coarsely chop onions. Transfer beans and onions to a large bowl. Dress with prepared sauce. Add mint leaves and peanuts. Toss to combine, and serve warm, at room temperature, or cold on a platter.