Begin by preparing all ingredients. Chop the tomatoes, scrub and wipe the potatoes, and peel and slice one of the onions into thin strips.

Heat the sunflower oil in a large frying pan or skillet over medium-high heat. Add the potatoes and sliced onion, stirring often. Fry for approximately 8 minutes, or until the potatoes and onions are lightly browned.

While the potatoes and onion are cooking, place the garlic, ginger, cilantro stems, the second onion (peeled), tomato paste, and Dijon mustard (if using) in a food processor. Blend until smooth, adding 1-2 tablespoons of water as needed to achieve a puree.

After the potatoes and onions are browned, add the blended spice mixture to the pan. Stir to ensure the potatoes are evenly coated, and fry for an additional 5 minutes, stirring occasionally to prevent sticking or burning.

Add the cumin, turmeric, smoked paprika, curry powder, and ground coriander. Mix well, and allow the spices to cook for 1 minute to release their flavors.

Add the chopped tomatoes and salt, stirring thoroughly. Cook for 8 minutes or until the tomatoes have broken down and integrated into the sauce.

Pour in 1 cup of water and mix. Reduce the heat to medium once the sauce begins to bubble, and simmer for approximately 15 minutes or until the potatoes are fork-tender.

Stir in the garam masala, lemon juice, and sugar (if using) for added balance. Cook for an additional minute, then remove from heat. Serve garnished with finely chopped cilantro leaves.