Peel potatoes and carrots and cut into 3/4-inch chunks. Place vegetables on a rack at least 1 inch above boiling water in a 5- to 6-quart pan. Cover and steam over medium heat until vegetables are tender when pierced, 8 to 10 minutes. Let stand until cool, or to cool quickly, immerse vegetables in cold water. When vegetables are cold, drain well.
Meanwhile, pour oil into a 6- to 8-inch frying pan over high heat. When oil is hot, stir in mustard and cumin seeds. Cover pan, remove from heat, and shake (holding lid on) until popping subsides, about 40 seconds. Add garlic and pepper.
In a wide bowl, combine seed mixture, yogurt, drained vegetables, and parsley; mix gently. Add salt to taste. Serve salad, or cover and chill up to 6 hours.