Toast the cumin seeds in an 8-inch skillet over medium heat until they darken slightly and become very fragrant, about 1 minute—keep them moving constantly to prevent burning, then transfer 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 just starting to turn golden, 5 to 7 minutes. Stir in half of the minced jalapeño, the ginger, and the garlic; cook until the aromatics are fragrant and beginning to brown, about 3 minutes.

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

While the soup simmers, place the mustard greens in a microwave-safe bowl and cook until they are wilted and tender, 3 to 4 minutes. The microwave is an efficient shortcut to tenderize the greens without diluting the soup's flavor. Transfer the greens to a colander to drain.

Prepare the sambal by tossing the tomatoes, lime juice, the remaining toasted cumin, the remaining jalapeño, and salt in a separate small bowl.

Remove and discard the bay leaf. Transfer a small portion of the soup to a blender along with the coconut milk and puree until the mixture is completely smooth, about 30 seconds. Stir the puree and the drained mustard greens back into the Dutch oven.

Return the soup to a brief simmer to marry the flavors, then season with salt and pepper to taste. Serve hot, topped with a generous spoonful of the tomato sambal for a bright contrast.