Toast mustard seeds in a small dry skillet over low heat, stirring constantly, until they start to pop, about 2 minutes. Transfer to a small bowl to cool.
Heat oil in a Dutch oven or soup pot over medium-low heat. Add onions, sugar and salt. Cook, stirring occasionally, until deep golden, 10 to 15 minutes. (Add 1 or 2 tablespoons water if mixture starts to burn.) Add garlic, ginger, jalapenos, coriander, curry powder and the toasted mustard seeds; cook, stirring, until fragrant, about 2 minutes more.
Add potatoes and water; bring to a simmer. Cover and cook until tender, about 10 minutes. Add cauliflower and carrots and simmer, covered, until tender and the sauce has thickened, about 10 minutes more. Adjust seasonings with salt and pepper. Transfer to a serving dish or individual plates and garnish with cilantro and toasted nuts. Serve with Cucumber-Mint Raita.
Tip: To toast chopped nuts: Place in a small dry skillet and cook over medium-low heat, stirring constantly, until fragrant and lightly browned, 2 to 4 minutes.