Place pumpkin seeds in a single layer in a heavy skillet and toast, stirring frequently, over medium heat until seeds are fragrant and slightly puffed, 3 minutes. Transfer to a separate plate and set aside to cool.
Place sesame seeds in same skillet. Toast, stirring frequently, until seeds become golden, about 4 minutes. Set aside to cool.
Grind pumpkin and sesame seeds together into a smooth paste in a spice grinder or with a mortar and pestle. Set aside.
Season chicken with salt and place in a large deep skillet. Add stock, cover skillet, and bring to a simmer over medium heat. Poach chicken, turning once, until juices run clear, about 20 minutes. Transfer chicken to a plate and cover with aluminum foil. Add onion, garlic, chile, tomatillos, and half the pistachios to poaching liquid and cook until vegetables are tender, about 10 minutes. Remove chile and discard.
Using a slotted spoon, transfer vegetables to a blender. Reserve broth. Purée vegetables, adding only enough broth, about 2 cups, to make a smooth sauce. Return sauce to skillet and warm over low heat.
Gradually whisk pumpkin–sesame seed paste into sauce, adding enough additional reserved broth to maintain a smooth sauce. Adjust seasoning with salt, add chicken, and heat through without allowing sauce to boil. To serve, spoon sauce onto four plates, arrange chicken over sauce and garnish with remaining pistachios.