In a large skillet, sauté pan, or wok, heat oil over medium-high heat until shimmering.
Add onion and plum tomato chunks, season with salt, and cook just until starting to soften, about 5 minutes.
Using a slotted spoon, transfer the vegetables to a plate, leaving the oil behind in the pan.
Add the cumin seeds and mustard seeds and cook until they begin to fry and sizzle in the oil.
Add 7 tablespoons (100ml) water along with the chana dal and moong dal, then bring to a simmer and cook for 5 minutes.
Add pureed tomatoes, return to a simmer, then reduce heat to low, and cook gently for 15 minutes to develop the flavor of the curry.
Stir in garam masala, cumin, sugar, and turmeric.
Return the onion and tomato chunks to the pan and cook until sauce is thickened and coats vegetables, about 2 minutes.
Season with salt to taste.
Serve with rice and/or flaky flatbreads.