Sweet potato, eggplant, tomato, and spinach curry

Ingredients

• 1/2 cup water, divided
• 1 teaspoon black mustard seeds
• 2 large cloves garlic, finely chopped
• 1 heaping tablespoon cumin seeds
• 1 large sweet potato or yam, peeled and cut into small cubes
• 1 15-ounces can diced tomatoes
• 1 large eggplant, cut into small cubes
• 1 bunch of spinach, blanched, dried, and chopped
• 1 Tablespoon Madras Sambar Powder or a good Masala mix / curry powder
• 1/2 teaspoon salt
• 1/4 teaspoon turmeric powder