I love this dish!
When I make this, I use a whole bag of frozen spinach (you could probably even use more), and a lot of cashews. I like to toast the nuts first in a dry skillet on the stovetop or in the oven, until they get fragrant and start to brown. You can serve the whole thing over brown rice.
All the ingredients for this are available in a regular grocery store. If you don't have something, like mustard seeds, you can just leave it out. Or if you don't have red lentils, it would be fine with ordinary lentils.