Heat oil over a medium-high heat. Toss in the mustard seeds; when they start spluttering, add the asafoetida, followed by the cumin seeds, turmeric powder and the green pepper. Add the tomatoes and ginger; stir and fry for a few minutes.
Add the potato, cauliflower florets, sugar and salt. Stir well and cook until the potatoes are cooked through and the cauliflower tender yet crunchy. (For a crunchier cauliflower, add it when the potatoes are just getting done but not quite finished cooking).