Preheat broiler with oven rack 3 in. from heat. Set yellow onion, poblanos, serrano, tomatillos, and garlic on a rimmed baking sheet. Broil, turning, until vegetables are browned to blackened all over, 15 to 30 minutes, moving them to a bowl as browned. Let cool.
Toast ancho chiles in a large pot over medium heat until fragrant, pressing down with tongs and turning occasionally, about 3 minutes. Turn off heat.
Peel and seed poblanos and serrano. Peel garlic. Whirl serrano, garlic, onion, tomatillos and any juices, and 1/2 cup pumpkin seeds in a food processor until very smooth. Pour into pot with whole anchos (they'll fall apart as they cook). Coarsely chop poblanos and add to pot.
Stir in hominy, 5 cups broth, 2 tsp. oregano, and the cumin. Cover, bring to a boil over high heat, then reduce heat and simmer, stirring occasionally, until cumin flavor is mellow, about 45 minutes. Discard any large pieces of ancho chile.
Stir zucchini into posole and simmer just until tender, about 5 minutes. Stir in more broth if you'd like a thinner soup.
Ladle posole into bowls and serve with remaining pumpkin seeds and oregano and other accompaniments to taste.