Preheat the slow cooker if necessary; see the manufacturer's instructions. Rinse the pheasant breasts with cold water, pat dry with kitchen paper and season well with salt and pepper. Top each breast with a few sage leaves, then wrap in pancetta slices until completely covered. Tie at intervals with fine string to keep the pancetta in place.
Heat the butter in a frying pan, add the shallots and fry for 4–5 minutes or until browned. Stir in the flour, then add the cider, stock and mustard. Add the apple and chestnuts and a little extra salt and pepper. Bring to the boil, stirring.
Arrange the pheasant breasts in the slow cooker pot. Pour the hot onion mixture over the top, cover with the lid and cook on low for 2½–3 hours or until the pheasant is tender and cooked through to the centre. Spoon on to plates, remove the string from the pheasant and serve with baby carrots.