Put the lentils, half the tomatoes, and the jalapeños, ginger, garlic, coriander, and turmeric in a large pot; add enough water to cover by 1½ inches. Cover, bring to a boil over high heat, then reduce the heat to medium and simmer until the lentils are almost tender, about 30 minutes.
Continue cooking the lentils while you put the oil and butter in a medium saucepan over medium heat. When the butter melts, add the cumin seeds, cinnamon stick, and cloves, and cook, shaking the pan occasionally, for 2 to 3 minutes. Add the onions and cook, stirring occasionally, until soft, about 10 minutes. Add the cayenne and 1 cup water; bring to a boil over medium-high heat and simmer for 1 minute, then turn off the heat.
When the lentils are fully tender, partially purée them with an immersion blender (or leave them whole if you prefer). Stir the onion mixture into the lentils along with the remaining tomatoes and the cilantro. Season liberally with salt and pepper. Simmer for 5 minutes, then turn off the heat and stir in the lemon juice. Taste and adjust the seasoning, and serve hot or warm over basmati rice, if desired. (Store leftover dal in an airtight container in the refrigerator for up to a week.)
