Set an oven rack about 6 inches from the heat source and preheat the oven's broiler to high. Spray a baking sheet with fat-free cooking spray.
Cut tomatillos in half and place cut-side-down on the prepared baking sheet with serrano pepper, jalapeno pepper, onion, and garlic.
Broil in the preheated oven until charred, about 20 minutes, turning peppers halfway through cooking time.
Remove vegetables from the oven and let cool completely, about 15 minutes. Remove stems from peppers and peel garlic.
Combine tomatillos, serrano pepper, jalapeno pepper, onion, garlic, cilantro, lime juice, and salt in a food processor and pulse to desired consistency. Store in an airtight container in the refrigerator for at least 30 minutes to meld flavors.