On a dry griddle or comal, roast chopped onion, chiles and unpeeled garlic. Toss every few minutes until everything is charred all over, about 10 minutes. Remove from griddle and peel garlic cloves. Place in blender jar.
Toast the sesame seeds over medium heat in a dry frying pan until golden and fragrant, around 5 minutes. Toast pumpkin seeds in the same way – they will puff up nicely. Place sesame seeds and pumpkin seeds in blender jar along with onions, chiles and garlic.
Meanwhile, in saucepan, cook tomatillos with enough water to cover until they are soft, about 10 minutes. Drain tomatillos and place in the blender with onion, chiles, garlic, sesame seeds and pumpkin seeds. Add epazote and about 1 ½ cups chicken stock. Process until very smooth, adding more stock if necessary to make a very smooth sauce.
In a large sauce pan, heat manteca and add the blended sauce. Cook over medium heat for about 5 minutes, stirring every so often. Slowly add 2-3 cups stock and continue cooking over medium low heat until the sauce thickens and covers the back of a spoon. Season with salt to taste.
In large frying pan, heat olive oil. Add smashed garlic clove and cook until oil is fragrant. Remove garlic. Season shrimp with salt and pepper and cook in frying pan, turning often, until pink, 2-4 minutes.
Serve a pool of pipián on the plate and arrange shrimp on top. Garnish with toasted pumpkin seeds. Serve with white rice.