You can roast the tomatillos and serranos on a comal or cast-iron skillet or in the oven. To pan-roast them, heat your skillet to medium-high. It needs to be quite hot before adding them, so be patient! Roast them until they are blackened and soft.
Add fresh onions, garlic, and cilantro to a food processor or blender with the serranos and tomatillos. Pulse everything until the consistency is smooth. If you like more chunks, stop pulsing earlier.