Place a piece of foil on a large (10-inch) skillet or griddle set over medium heat. Roast the tomatillos and onion directly on the foil, turning occasionally, until soft and blackened in spots, 10 to 15 minutes. Set aside to cool.
Remove the foil from the skillet and add the pumpkinseeds to the hot pan. When the first one pops, stir constantly until all have popped from flat to round, about 5 minutes. Remove to a bowl to cool. Add the chiles to the skillet and toast, flattening them with a spatula, until fragrant—about 2 seconds. Flip and toast the other sides, then remove to a plate to cool.
Scoop the onion, tomatillos, chiles and pumpkin seeds into a blender. Add the ground spices and 1 cup of broth or water. Blend to a smooth puree and press through a medium-mesh strainer.
Heat 1 tablespoon of the oil in a large (4-quart) saucepan over medium-high. Add the puree and stir constantly until it reduces into a thick paste, 5 to 7 minutes. Lower the heat to medium, add the remaining 1 ½ cups of fish stock and simmer, stirring occasionally, so the flavors blend and the sauce thickens to the consistency of a light cream sauce (about 45 minutes). Taste and season with salt, usually about 2 teaspoons.
Measure the remaining 1 tablespoon of oil into a large (10-inch) skillet set over medium-high. Generously sprinkle the fish fillets all over with salt and lay them skin side down in the hot oil. Cook until the skin is browned and crispy, about 3 minutes. Turn the fillets over and cook until the fish is as done as you like, about 5 minutes longer for medium. Spoon a generous portion of the pipian onto six plates, and top each with a piece of the fish crispy skin side up.