How to Make It
Heat a grill pan over medium-high heat. Coat pan with cooking spray. Arrange white and red onions on pan; grill 10 minutes, turning occasionally. Place grilled onions in a large bowl. Add green onions to pan; grill 2 minutes on each side. Add to onion mixture; cover with plastic wrap. Let stand 10 minutes. Add oil, juice, and salt; toss to coat. Add arugula, radishes, and cilantro; toss to coat.