I particularly wanted to tackle this as the tomatillo is a new ingredient for me.
They have a papery husk and are a bright green colour. They are related to the gooseberry which makes sense when you consider the husk, and the flavour certainly has some of the quality of a gooseberry, even though their size and appearance when the husk is removed is exactly that of a green tomato.
Recipes for tomatillo salsa come in three main varieties; raw, cooked and finally the roasted.
Raw Tomatillo Salsa (Salsa De Tomate Verde Cruda)
Typically the raw variety calls for about a half dozen tomatilloes, onion, garlic, coriander and chili (generally 2 serrano chilis for that quantity of tomatilloes). This is then blended along with a small quantity of water -enough to get the blending started easily - and seasoned with salt pepper and some lime juice. Alternately the diced onion could be added after the tomatilloes have been blitzed.
This gives a very vivid bright green salsa with a very interesting fruity taste. It is not at all like the more common cooked variety which has an earthier taste more reminiscent of vegetable than fruit.
It is suggested that this salsa is best eaten on the same day as it is made. While this may be true, I found it kept very well in the fridge (I kept it in a zip-lock bag) and I was surprised at how well it kept it's colour, it is still the same vivid green almost two days later. One issue though is that the salsa does have a tendency to separate slightly and it is not possible to keep the same smooth consistency as the cooked salsa.
I used the flesh of two serrano chilis. There seems to be no fixed rule as to whether jalepenos or serranos are used. The flesh of two serranos I used in this salsa was somewhat wimpy, however for the raw version of the salsa I think it is important, not to hide the fruity flavour of the tomatillo with too much heat.
Cooked Tomatillo Salsa (Salsa De Tomate Verde Cocida)
The more common version of the salsa requires that the tomatilloes be poached first, along with the chili, until they are soft. This should take just a few minutes, certainly less than ten. The tomatilloes will change colour becoming a much duller green gloden colour. The cooked tomatilloes are then blended with the chili, garlic, coriander, onion and seasoned.
This time I used jalepenos which I poached along with the tomatilloes, the flesh and seeds of a single jalapeno were more than enough heat for a quantity of about six tomatilloes.
It should not be necessary to add any water to the blender, the tomatilloes and chili will be soft enough to pulse easily. After the tomatilloes chilli and garlic have been blended to a rough purée add the finely chopped onion and the coriander and season, as before, with salt pepper and lime juice.
Occasionally you will see a recipe for tomatillo salsa which calls for sweated onion to be added rather than raw finely diced onion. While this would fit with the softer velvety texture of the cooked tomatilloes, it seemed like more trouble than I was prepared to go to. Another occasional ingredient is some ground cumin. While I normally add some cumin to a tomato salsa, the vast majority of recipes fora tomatillo salsa omit it and I would agree, I'm not sure it adds anything.
Roast Tomatillo Salsa
There is also a subset of the cooked tomatillo salsa recipes where the tomatilloes are either roasted, grilled or charred on a comal. These recipes also commonly call for a large chili such as a poblano or anahiem be charred and added to the salsa.
I have yet to try this version of the sauce, however I have a couple of poblanos and some tomatilloes set aside especially for this purpose and I'll do a seperate post on this version of the sauce sometime soon.