Preheat your broiler or oven. Place serrano peppers and tomatillos on a baking sheet. Broil until they start to blacken, turning once, about 5 minutes per side.
Let them cool slightly, then add the peppers, tomatillos, cilantro, onion, lime juice, and a pinch of salt to a blender or food processor. Include garlic if desired.
Pulse until the mixture is mostly smooth but still has some texture.
Taste and adjust salt or add more lime juice if needed.
Transfer to a bowl and refrigerate for at least an hour to let the flavors meld.
Serve chilled or at room temperature.
Notes
Roast tomatillos and serrano peppers to deepen their flavors before blending.
• Wear gloves when handling serranos to protect your skin.
• Start with fewer peppers and add more to taste for the desired heat level.
• Use fresh lime juice for the best flavor.
• Let the salsa chill in the fridge for a few hours to enhance the flavors.