Toast the pumpkin seeds in the large skillet over medium heat (or in the oven on the baking sheet at 350° F/175° C), stirring and tossing, until they being to smell fragrant. Remove from heat and cool.
In the 4-quart saucepan, add the whole tomatillos, garlic, and chiles and cover with water. Bring to a simmer over medium heat and cook until the tomatillos are cooked all the way through, about 10 minutes. Remove from heat.
In the food processor, grind the pumpkin seeds until you have a coarse powder. Add the cooked tomatillos along with the garlic, chiles, and cooking liquid. Add the onion, cilantro, and salt, and puree until smooth.
In the saucepan, warm the canola oil over medium heat, then pour in the tomatillo puree and add the chicken broth. Stir well and cover with the lid slightly ajar. Let the sauce simmer until it has thickened and changed color, about 35 minutes.
Shrimp
About 5 minutes before the sauce finishes, warm the remaining canola oil in the large skillet over medium heat. Add the garlic and sauté just until it releases its fragrance, about 1 minute. Add the shrimp and cook while stirring, until the shrimp turns pink, about 4 to 5 minutes. Don’t let them go much longer or they’ll be tough. Transfer to a serving bowl and squeeze some lime over it. Serve with the pipián verde.