Sauteed Okra andTomatoes

1 T whole mustard seeds
1/2 tsp whole cumin seeds
1/2 t ground coriander
2 T extra-virgin olive oil
1 sm red onion, cut into 1/2" thick wedges
1# fresh okra, stems & ends trimmed
3 medium tomatoes, seeded & cut into1/2" thick wedges
coarse salt & pepper

In a medium bowl, combine mustard sees, 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 heat, 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.