Heat the duck fat in a large, heavy stockpot. Add the chopped onions and cook over moderately high heat, stirring often, until softened, about 7 minutes. Add the garlic and cook, stirring, until fragrant, about 3 minutes. Stir in the greens by the handful, adding more as the previous batch starts to wilt. Add the chipotles and season lightly with salt and pepper. Cover and cook over low heat, stirring often, until the greens are very tender, about 1 hour.