Preheat oven to broil. Line a rimmed baking sheet with foil. Place the husked tomatillos on the baking sheet. Broil for 5 minutes; flip them over and broil about 4-5 minutes more. The skins will begin to char, but watch carefully to prevent then from completely burning.
While the tomatillos cool, chop your cilantro, red onion, garlic, and serrano pepper. Zest and juice your lime(s).
Place all the ingredients in a blender and pulse until combined. It should be a bit chunky, so be careful not to overblend.
I used the a different tomatillo salsa recipe in the Muy Bueno cookbook to figure out how to roast the tomatillos. In my haste, I misread the instructions and peeled them after I roasted them (the recipe roasted tomatillos, some peppers and garlic together and then had you steam and peel only the peppers). I didn't realize my mistake until typing this out. For those of you who ate the salsa I made, I wanted you to know that... you can choose whether to remove the peel or not. I would guess the skin will slightly change both the taste and the texture, perhaps for the better!
Source: Recipe slightly adapted from The Love and Lemons Cookbook