Heat the oil in a large skillet over medium heat.
Add the cumin and mustard seeds. Cook until the cumin seeds change color, 1 to 2 minutes.
Add the onion and cook until it is translucent, 5 to 6 minutes.
Meanwhile, in a blender, blend the tomato, garlic, and ginger into a coarse puree.
Add the tomato mixture, turmeric, cayenne, coriander, ground cumin, and garam masala to the skillet. Cook, stirring occasionally, until the puree thickens and the garlic is fragrant, 5 to 6 minutes.
Add the potatoes, salt, and water. Cover the skillet and cook for 10 to 11 minutes.
Add the peas. Taste and adjust the seasonings.
Reduce the heat to medium-low. Simmer for another 10 to 12 minutes, or until the potatoes are cooked to your preference.
Add additional water if the potatoes start to stick or if you prefer more curry. Add the cilantro and lemon juice (if using).