They may be tiny, but seeds pack a powerful nutritional punch.
Place chicken in a 4-quart pot, and add water to cover by 1 inch. Bring to a gentle simmer over medium heat, and cook until chicken is cooked through, about 15 minutes. Remove from heat, and let stand 15 minutes. Transfer chicken to a plate, and cover.
Place serrano and poblano chiles directly over the flame of a gas-stove burner on high heat. Roast, turning with tongs, until blackened all over. (Alternatively, broil chiles on a baking sheet, turning often, until skin has charred.) Transfer chiles to a bowl, and cover with plastic wrap. Let stand for 15 minutes. Peel chiles, and remove stems, seeds, and ribs.
Process roasted chiles, onion, garlic, cilantro leaves, 1/2 cup pepitas, cumin, oregano, and 1/2 cup stock in a blender until a coarse paste forms.
Heat oil in a 4-quart pot over medium heat. Add paste, and cook, stirring constantly, until very thick, about 9 minutes. (Reduce heat if mixture begins to scorch.) Whisk remaining 1 1/2 cups of stock into paste until incorporated. Let mixture cool slightly, then transfer to blender. Add salt and lime juice. Process until sauce is smooth.
Place chicken in a clean 4-quart pot, and cover with sauce. Cook over medium-low heat until just heated through, about 10 minutes. (Do not simmer.)
Transfer chicken to a cutting board, and cut into 1/2-inch-thick slices. Distribute evenly among plates, spoon sauce on top, and garnish with cilantro sprigs and remaining 2 tablespoons pepitas. Serve with rice if desired.
