Do this while your lentils cook if you're pressed for time.
Heat a not too big skillet over medium low heat. Add the oil. When the oil starts to shimmer add the onions. Sweat the onions, stirring occasionally, until soft and translucent. This takes around 5 minutes or so.
Toss in the diced jalapeño and cook for another minute.
Add the cumin seed and mustard seed. This is the tricky part. You want the oil to form little bubbles around the seeds. See them all move just a little. Like a whole spice rave in the pan. Let the spices party for about 30 seconds.
Toss in the tomato and the cilantro and continue cooking until the tomato is just warmed through.
Fold the mixture into the dal. Add the salt. Taste. You are going to need more. Creep up on the salt profile that works for you.
Serve as a side dish with tandoori chicken or the curry of your choice. Or serve it as a main with an Indian flatbread. I'm in heaven with this dal and a paratha or chapati.