A couple of weeks ago I was trying to replicate the green sauce from Taquiera Arandas, ended up buying like 20 bucks worth the vegetables/peppers to try and figure it out.. I tried about 3-4 different 'formulas' (and even tweaked them further to try and get close to it) but I never really did.. I made some stuff that was an acceptable substitute, but it just wasn't the same, not the flavor, not the consistency..
I was convinced the recipe was wrong, my wife asked them how they make it and they told her they use serranos and tomatillos and that's it.. All the sauces I managed to make using tomatillos ended up being sweet (the real stuff was not sweet at all, it wasn't tangy and it wasn't salty.. the only way for me to counter the sweetness was with tangy flavors or salt).
Okay so just now, I was making a concentrated spicy liquid to add to the buttermilk brine for hot wings, once I put it all in the blender and blended it up to make a smooth sauce, I noticed immediately it looked exactly like the sauce from that place, then I tasted it.. Tasted just like it too.. Side by side you wouldn't be able to tell the difference between theirs and mine..
The reason I couldn't replicate it is because they lied <_<. All I did was take 5 serranos and cut them lengthwise, then diced them up, and threw them in like 2 cups of water, along with a jalapeno split lengthwise and then cut into slices, about 5-6 cloves of garlic, a dash of salt and the tips to the chicken wings, and boiled it all down, til there was only about 1/2 cup of liquid left, removed the wing tips and then blended everything that was left.