Heat a wide 4 - 5 quart heavy pot over medium heat until hot, about 5 minutes, then toast the pumpkin seeds, stirring constantly, until slightly puffed and beginning to pop, 3 to 6 minutes. Transfer to a plate and let cool.
While seeds are cooling, discard husks from the tomatillos and rinse under warm water to remove any stickiness.
Coarsely chop tomatillos, then puree in a blender with onion, garlic, chiles (to taste), epazote, salt, 1/2 cup cilantro, and 1/2 cup water until very smooth.
Grind pumpkin seeds to a fine powder in small batches, then stir together with 2 cups water in a medium bowl.
Heat oil in cleaned pot over medium heat until hot but not smoking, then cook pumpkin seeds mixture, stirring constantly, until thickened to a thick consistency that will pull away from the side of the pot, 10 to 15 minutes, careful to not brown, or start to burn.
Add tomatillo puree and simmer, stirring, 2 minutes.
Stir in the 1 1/2 cups water, and simmer, uncovered, stirring frequently, for 10 minutes (large streaks or pools of green oil from the pumpkin seeds will appear on the surface).
Just before using, puree remaining 1/2 cup cilantro with 1/2 cup water in blender until smooth and add to the sauce. If necessary, thin with a little more water and return to a simmer.