Toast the cumin seeds in a small skillet over medium heat until they smell fragrant and darken slightly, about 1 minute, then transfer them to a small bowl.

Heat the oil in a Dutch oven over medium heat until it shimmers. Add the onion and salt and cook, stirring occasionally, until the onion is softened and lightly browned, 5 to 7 minutes.

Stir in half of the jalapeño, the ginger, and the garlic. Cook until fragrant and the aromatics just begin to brown, about 3 minutes.

Add the lentils, broth, bay leaf, turmeric, and half of the toasted cumin. Bring the liquid to a simmer, then reduce the heat to low, partially cover, and simmer until the lentils are completely tender, 40 to 50 minutes.

Wilt the mustard greens in a microwave-safe bowl until they are tender, about 3 to 4 minutes. Transfer them to a colander to drain well.

Prepare the garnish by tossing the chopped tomatoes with the lime juice, the remaining toasted cumin, the remaining jalapeño, and a pinch of salt.

Discard the bay leaf. Transfer about 3/4 cup of the soup to a blender, add the coconut milk, and puree until completely smooth, about 30 seconds, then return the mixture to the pot.

Stir the drained mustard greens into the pot and bring the soup back to a brief simmer.

Season with salt and pepper to taste. Serve hot, topping each individual bowl with a generous spoonful of the tomato mixture.