Cut bell peppers in half lengthwise; discard seeds and membranes. Place bell pepper halves, skin sides down, on grill rack coated with cooking spray. Grill 5 minutes on each side or until blackened. Place in a zip-top plastic bag; seal. Let stand 10 minutes. Peel and cut bell peppers into strips.
Place onion on grill; grill 7 minutes on each side or until tender. Cool 5 minutes; cut onion slices in half.
Combine bell peppers, onion, arugula, and tomatoes in a large bowl; toss gently to combine.
Combine shallots and the remaining ingredients, stirring with a whisk. Drizzle dressing over salad; toss gently to coat. Serve immediately.