Cut potatoes (skin on) into 1 1/2- to 2-inch chunks. Add to boiling water and cook until tender but still firm. Drain well.
Tear Swiss chard (including stem) into large pieces. Place in boiling water for 3 to 4 minutes. Pour into a colander to drain.
In a large bowl, combine potatoes, Swiss chard and onion.
In a small bowl, combine garlic, vinegar, oil and red pepper flakes. Pour over potato mixture and gently toss. Add salt and toss again. Refrigerate until chilled.