Heat the olive oil in a deep, wide pan or Dutch oven over medium-high heat until it shimmers. Add the cumin seeds and mustard seeds and toast until the mustard seeds begin to pop and the cumin turns a shade darker, about 30 seconds.

Reduce the heat to low, then add the garlic and ginger, stirring briefly until fragrant. Stir in the tomatoes and salt, increase the heat back to medium, and cook until the tomatoes soften and begin to break down, about 3 minutes.

Stir in the coriander, cumin, red chili, and turmeric. Add the potatoes and 2 cups of water, then bring the mixture to a gentle boil.

Lower the heat to medium-low to maintain a continuous simmer. Cover the pan and cook until the potatoes are fork-tender, about 15 minutes.

Uncover the pan and mash a few of the potatoes with the back of a spoon to release their starch, which will naturally thicken the sauce. Increase the heat to medium-high and continue to cook without a lid until the sauce reaches your desired thickness, about 5 minutes.

Add the greens and stir until they are completely wilted into the sauce. Adjust the salt to taste, garnish with cilantro, and serve hot with flatbread.