In a small bowl, combine mustard seeds, cumin seeds, and coriander; set aside. Heat oil in a large skillet set over medium heat. Add onion, and cook, stirring, until soft, about 3 minutes.
Raise heat to medium-high; add spice mixture, okra, and 1/2 cup water. Cook, stirring, until okra is bright green and just tender, about 6 minutes, adding more water if skillet becomes too dry.
Add tomato wedges, and cook until just heated through, about 1 minute. Season with salt and pepper. Serve immediately.