Place the tomatillos in a 2-quart saucepan, cover with water and bring to a boil. Cook for about 6 minutes, until they are soft.
Drain, let cool briefly, and cut into quarters.
While the tomatillos are cooking, lightly toast the sesame seeds and pumpkin seeds in a dry skillet over medium heat, stirring, about 5 minutes, until starting to brown.
Place the tomatillos and seeds in a blender with the chile, garlic, onion, lettuce, cilantro and stock. Blend until smooth, adding additional stock as needed.
Transfer the mixture to the saucepan over low heat and cook for about 5 minutes, until the consistency of ketchup is reached.
Remove from the heat and season with salt and pepper. The molé verde sauce can be refrigerated for several days before using.