In a large Dutch oven over medium, heat the oil until shimmering. Add the cumin seeds and cook, stirring, until fragrant, 30 to 45 seconds. Stir in the onion, cover and cook, stirring occasionally, until begins to brown, 3 to 5 minutes.

Add the ginger and garlic, then cook, stirring, until fragrant, about 30 seconds. Stir in the paprika, cayenne, turmeric and garam masala, then cook until fragrant, about 30 seconds.

Add the sugar, tomatoes with their juice and 2 cups water, then bring to a simmer, stirring occasionally. Stir in the potatoes and ¾ teaspoons salt.

Return to a simmer, then cover, reduce to medium-low and cook, stirring occasionally, until a skewer inserted into a potato meets just a little resistance, about 20 minutes.

Uncover and continue to cook, stirring often, until the potatoes are completely tender and the sauce clings lightly, another 5 to 7 minutes.

Remove from the heat, stir in the peas and let stand, uncovered, until heated through, about 5 minutes. Stir in the cilantro, then taste and season with salt and pepper.