Bring a large pot of lightly salted water to a boil. Add the carrots and potato; cook for 5 to 6 minutes or until edible but not mushy. Add the peas and corn to the boiling water in the final minute. Drain.
While the vegetables are boiling, heat the oil in a large frying pan over medium heat. Add the onions; cook and stir until the onions have become translucent. Add the tomatoes to the onions and the water. Season with turmeric, chilli powder, cumin, coriander powder, mustard seeds and salt. Taste and adjust seasoning as desired, especially the salt.
Mix in the vegetables and increase the heat to high. Cook for 5 to 7 minutes, stirring constantly. Reduce heat to medium and simmer for 7 minutes. Stir in the coconut milk and cook for 3 minutes ensuring the milk doesn’t curdle.
Add the spices and stir well, making sure it is to your taste (this is especially important regarding the salt).