In a large skillet or wok, melt butter over medium-high heat, and cook and stir cumin seed, chile pepper, garlic, and turmeric until fragrant, about 2 minutes.
Stir in the chopped mustard greens and spinach a little at a time, adding the tougher parts first (the stems and thicker leaves). Continue to add greens, and cook and stir until all greens have been added and all are thoroughly wilted. Stir in the cumin, coriander, and salt. Cover; reduce heat and simmer until greens are tender, about 10 minutes, adding water as needed to keep the greens moist.