In a high-sided saucepan over medium heat, add the tomatillos, serrano peppers, garlic, and enough water to cover the tomatillos. Simmer for 10 minutes until the tomatillos are soft.
SImultaneously, in a separate pan over medium heat, add the pepitas and brown them for 3 minutes, moving them often until lightly browned. Set the pumpkin seeds to the side.
Drain the tomatillos, serranos, and garlic then place them in a blender. Blend smooth.
Add the pepitas, white onion, romaine leaves, cilantro, parsley, and salt to the blender and continue to blend until smooth.
Add the canola oil to the high-sided saucepan from earlier (or use a large saucepot), and heat the pan over medium heat.
Pour the sauce from the blender into the saucepan, then add the chicken broth. Stir well, then cover and lower heat to medium-low.
Simmer the sauce for 15 to 20 minutes, stirring 3 or 4 times during that time.
Once the sauce is to your preferred thickness, remove it from the heat and allow it to cool slightly. Use immediately.