Heat oil in a large frying pan over medium-high heat. Add the mustard seeds (cover with a splatter screen if possible) and as soon as they start popping reduce heat to to medium and add in the remaining spices (not garlic). Stir in the onions, and garlic. Cook until the onions are tender. Mix in the potatoes and "fry" them. Mash a few potatoes as this will add a thickness to the gravy. Add the peas, water, tomatoes, sugar, and salt into the vegetable mixture. Continue cooking about 10 minutes or until tomatoes have cooked down (about 10 minutes). Garnish with cilantro.