In a large skillet with high sides or in a large saucepan over medium-high heat, toast the pumpkin seeds, cumin seeds, and oregano, if using, until fragrant, 3 to 4 minutes. Toss frequently to make sure they don’t burn. Remove from the heat and transfer to a spice grinder or a blender and process until finely ground.
In the same skillet, heat the oil over medium-high heat. Add the onion, tomatillos, garlic, and jalapeños and cook until slightly browned, 4 to 5 minutes, tossing a couple of times but not too much. Place the vegetables in a blender or food processor, then add the broth, cilantro, parsley, epazote (if using), and salt and process until puréed.
Pour the mixture back into the skillet and add the ground pumpkin seed mixture. Let simmer gently until the flavors are melded, stirring occasionally, about 15 minutes. Serve immediately. (Leftovers—if there are any—can be refrigerated in an airtight container for up to a couple of days.)