Purée onion, chiles, and garlic with 1/2 cup chicken broth in a blender.
Heat 1 tablespoon oil in a 4-quart heavy pot over moderately high heat until hot but not smoking, then brown pork, stirring and breaking up clumps with a fork, just until no longer pink, about 4 minutes. Transfer to a bowl with a slotted spoon. Pour off all but 1 tablespoon fat from pot.
Add remaining 3 tablespoons oil to pot and heat over moderately high heat until hot, then carefully add purée (it will spatter), cumin, and salt. Cook, stirring frequently, until mixture is thickened and most of liquid is evaporated, about 10 minutes.
Add pork, hominy, cilantro, and remaining 1 1/4 cups broth and simmer gently, uncovered, stirring occasionally, 10 minutes. Serve chili sprinkled with pumpkin seeds and cheese.