Heat oil in a soup pot until shimmering. Sauté onion in oil for 2-3 minutes then add ginger, mustard seeds, cayenne, coriander, cumin, and fenugreek. Sauté another 5 minutes more.
Add potatoes, carrot, zucchini, tomato sauce, and water. Bring to a simmer and cook for 15-20 minutes. Then add cabbage, coconut milk, peas, and basil. Simmer 5 minutes more, until vegetables are tender.
Stir in lemon juice, then taste and adjust salt or spices as needed. If adding more spices, sauté them in a little oil first. Serve hot.