Whether you're a chef, or just curious, this is a recipe I really recommend. It's easy and quick, so just try it and you will never want to buy that brownish-looking bottled or canned salsa again. My mother and I have made it thousands of times, so it's completely foolproof. :) Don't be shy and enjoy.
Step 1: Ingredients.
Tomatillo (750 grams or 1.6 lb)
Also called tomato or green tomato (tomate verde in Spanish). This is the main ingredient as it's the body and color of the salsa. They are bright green tomatoes covered in a paper-like dull husk. They are really easy to get in Mexico and Latin America, and I've heard that you can also get them in the United States (I'm sorry but I don't know about anywhere else :(). See the first picture if you don't know them.
Serrano pepper (10 to 30 peppers, up to you)
Serrano peppers (or chile serrano in Spanish) are the original peppers used in salsa verde (see note below). They're a tiny pepper, only 1" or 3" long (50-75mm). They're 2 or 3 times hotter than the popular jalapenos, so 10 peppers should do a soft salsa, while 30 peppers will do a spicy one... but don't be afraid to experiment, in fact I advise you to. I'm using around 25 peppers, which is what I discovered that I like the most.
Serranos come in many colors, but use green anytime you can in order to keep the salsa verde, well... verde.
Note: I've read some recipes that use jalapenos instead of serranos. Honestly I've never seen anyone (nor friends or restaurants) using jalapenos in salsa verde so I don't know where that came from. I would much rather use serranos, but if you can't find them... I guess that jalapenos could make a good replacement.
Coriander (a bunch or handful)
You will need a good bunch of coriander, also known as cilantro or chinese parsley. Coriander is a green leafy herb, with a distinctive strong smell and taste. I like to use a complete bunch since I really enjoy the herbal taste given by coriander, but some people use barely a few spoons. I recommend the whole bunch, but again, don't be afraid to experiment.
Onion (half an onion)
Use about half a big onion. Everybody knows onions, right?
Garlic (two large cloves)
It gives salsa a really good taste, and it's not even noticeable. If you don't like it too much, well leave one out.
Salt (half a teaspoon)
Add salt to taste. I think that it's about a teaspoon but the fact is that I never measure it. Just taste it and add salt accordingly.
You will need to add some water for consistency. Half a cup may do, but you will have to add less if the tomatillos are too watery.