Cook potatoes in boiling water 20-25 minutes, or until tender. Drain and cool to room temperature. Quarter the potatoes and set aside.
Grill the onion slices over high heat until lightly caramelized.
In a hot, dry skillet, toast the coriander and cumin seeds until aromatic, about 30 seconds. Add oregano and lightly toast for about 20 seconds, watching for burning. Remove spices from skillet immediately.
In a large bowl, combine all ingredients and lightly toss.