In a large skillet with high sides or in a large saucepan over medium-high heat, toast the 1 cup shelled pumpkin seeds, 1 teaspoon cumin seeds, and ½ teaspoon dried 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 1 tablespoon vegetable or olive oil over medium-high heat. Add the 1 white onion, 5 tomatillos, 5 garlic cloves, and 2 jalapeño peppers 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 2 cups chicken stock, 1 cup packed coarsely chopped cilantro leaves, 1 cup coarsely chopped flat-leaf parsley leaves, ½ cup fresh epazote (if using), and 1 teaspoon 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.
