There are a couple of types of recipes for rajas. The most common is rajas in crema and you sometimes see recipes for rajas composed of different chiles presented as a kind of salad. This version is in a light tomato salsa/broth.
Rajas refers to strips of chile, generally the larger chiles like Poblano or Anaheim. I used poblano. The quantites here aren't very exact, I used 2 poblanos and a tennis ball sized onion, along with over a half cup of diced panela cheese. Just keep in mind that the final dish should have quite a lot of broth for something that is served as a side dish.
Roasting the peppers
The first job is to roast the chiles. To did this I simply lay the chiles on the gas ring and turn it until they are blackened all over. The job can be done just as well under a hot grill (or even on a charcoal grill, though it seems like unreasonable amount of hassle to me). Once the chiles are black all over they are placed into a zip lock bag or into a covered bowl to steam and to cool enough to allow them to be handled. When they are cool enough to handle simply rub off the blackened skin, remove the stem seeds and membranes and cut the chile into slices slightly wider than 1/2cm.
There is nothing difficult in this and nothing that will be unfamiliar to anyone who has roasted a sweet red pepper. The only real knack is having the patience and confidence to wait until the chile is blackened all over before removing it from the heat.
Unlike sweet peppers, poblanos in particular, can be oddly shaped and fold over on themselves. This can make roasting them awkward, so it's best to pick pepper that are as symmetrical as you can find.
While the peppers are steaming saute a onion cut into similar sized strips. Fry the onions until they are translucent and just starting to pick up some colour. At this point add the strips of poblano and toss both together.
The salsa is made by blending a couple of cups of water with a plum tomato, a clove or two of garlic, some onion, a serrano chile and some oregano. Blend this to a smooth loose sauce and add to the poblano and onion. Season and allow everything to come to a simmer.
The cheese I use in this dish is Queso Panela. This is a white rubbery unaged cheese. It doesn't melt easily, you often see ricotta mentioned as a substitute, however panela is firmer. It works well here as it can easily be cut into small neat dice which hold their shape well and don't melt into the broth.
The dish will need some time to cook out and to allow the raw garlic, onion and tomato in the sauce to cook through and mellow. 15 or 20 minutes at a low simmer is plenty, the dish reheats well and the flavours are better after some time to meld.
I had a supply of good chicken stock frozen in cubes in the freezer so I added a couple of those. They were an improvement but are not absolutely necessary.
The dish has a fair amount of heat from the serrano in the salsa (I don't remove the seeds from a serrano. I find that a serrano without the seeds, while not as hot, is quite boring and offers nothing except just heat, the grassy and fruity taste you get with the seeds included is much more interesting. It is better to use less and include the seeds than to use just the flesh. Alternatively you could take a step down the heat scale and use a jalapeno rather than a serrano.) There is a smokey taste from the poblano and sweetness from the onion.
It works well as a side dish and as an accompaniment to tacos.