Place yogurt and ½ cup water in a large bowl. Whisk gently with a fork until combined. Set aside.
Heat oil in a pan over medium-high heat. Splutter mustard and fenugreek seeds. Add ginger, garlic, red chillies, and curry leaves, and sauté for a minute, stirring occasionally. Add onion and green chillies, and sauté for another 2 minutes, stirring occasionally, until the onions turns brown.
Add tomatoes, and mix well. When the tomatoes start to break down, lower the heat, and add ground turmeric. Saute for a few seconds, stirring continuously. Add spinach and salt. Turn up the heat, and cook for 4-5 minutes, stirring occasionally.
Remove pan from heat. Stir in yogurt. Place the pan over low heat. Warm the curry for about 4 minutes, stirring continuously.
Serve warm with rice.
Notes
Spinach can be substituted with red palak. Do not curdle yogurt.