Preheat oven to broil.
Remove husks from tomatillos and wash skin. Cut tomatillos in half and add to a bowl.
Remove stem and cut serrano peppers in half. Add to the bowl with chopped onion, garlic, olive oil, and a pinch of salt. Toss well.
Pour salsa verde ingredients onto a baking sheet, then bake in preheated oven for about 7 minutes, until tomatillos start to soften and brown.
Remove from oven and transfer ingredients to a blender. Add cilantro and blend until smooth. Season with salt to taste.