Chicken with Pumpkin Seed–Tomatillo Sauce (Pollo en Pipián Verde)

Ingredients

• 1 (3–4-lb.) whole chicken, cut into 8 pieces
• 4 serrano chiles, stemmed
• 4 cloves garlic, peeled
• 1 1⁄2 medium white onions, cut into quarters
• 1 lb. tomatillos, husks removed, rinsed
• 1 cup roughly chopped cilantro
• 4 large leaves romaine lettuce, roughly chopped
• 1 1⁄3 cups raw, unhulled pumpkin seeds
• 1⁄2 cup roasted unsalted peanuts
• 1⁄4 cup sesame seeds
• 1⁄4 cup canola oil