Peel potatoes and cut into 1/2- inch cubes; place in a bowl of water. Separate cauliflower into flowerets about the same size as the potato cubes. Set aside.
Mix mustard with fenugreek in a small bowl. Mix ground spices together in a small bowl, and add some salt. Place all ingredients near the stove.
Heat oil in a pan until very hot. Add whole seeds all at once, keeping face away from pan. Have a lid ready to cover pan, if needed. As mustard seeds turn gray, lower neat to medium and add ground spices. Cook for 1 or 2 seconds, then stir in garlic, then tomato and green chilies. Cook for about 5 minutes.
Drain potatoes and add to pan, along with cauliflower; cook for 5 minutes, stirring to coat vegetables with spices. Add 1/2 cup water and 1/2 cup cilantro. Cover and simmer until vegetables are tender. Check water level frequently, and add more as needed.
Taste for salt, add lemon juice, stir well, and serve garnished with remaining cilantro.
From the San Francisco Chronicle