In a skillet, heat oil over medium-high heat for 30 seconds. Add turkey and brown on all sides. Transfer to slow cooker stoneware.
Reduce heat to medium. Add onions to pan and cook, stirring, until softened, about 3 minutes. Add garlic, cloves, cinnamon stick, salt and peppercorns and cook, stirring, for 1 minute. Transfer mixture to blender. Add tomatillos, chocolate and 1⁄2 cup (125 mL) of the stock and process until smooth.
Pour sauce over turkey, cover and cook on Low for 8 hours or on High for 4 hours, until juices run clear when turkey is pierced with a fork or meat thermometer reads 170°F (77°C).
Half an hour before recipe has finished cooking, in a heatproof bowl, soak dried chiles in boiling water for 30 minutes, weighing down with a cup to ensure they remain submerged. Drain, discarding soaking liquid and stems and chop coarsely. Transfer to a blender.
Add cilantro, remaining 1⁄2 cup (125 mL) of the chicken stock, chili powder and jalapeño pepper and purée.
Add to stoneware along with mild green chiles, if using, and stir gently to combine. Cover and cook on High for 30 minutes, until flavors meld.
Note: Works best in a large (minimum 5 quart) slow cooker.