Preheat the oven to 180C/350F/Gas 4.
Using a sharp knife take the very top of the garlic bulb off (this will make it easier to get the roasted cloves out).
Skewer the baking potatoes and place in the oven with the garlic.
Cut the chicken breasts in half horizontally, then using a meat tenderizer bash each piece so it is nice and thin.
Season the flour generously and stir in the sage. Place on a shallow plate.
Put the breadcrumbs onto a separate shallow plate. Beat the egg and pour into a shallow bowl. Line the three plates/bowls up in a row.
Lightly dust the chicken in the flour, then dip in the egg and finally coat with the breadcrumbs. Keep refrigerated until ready to use.
After one hour remove the garlic bulb and allow to cool enough to handle. Then squeeze out the soft cloves.
Leave the potatoes in a further 30 minutes until cooked through.
When the potatoes are cooked cut in half and scoop out the flesh (reserve the skins). Melt the 50g/2oz butter and 4tbsp cream in a large heavy based pan.
Next pass both the potatoes and the garlic through a sieve into the pan with the butter and cream. Season with salt and white pepper. Mix well and keep warm.
Heat a small heavy based pan, add the sherry and reduce by half. Whisk the mustard and 150ml/5fl oz cream into the reduced sherry and reduce until thickened. Just before serving stir in the chives.
Meanwhile melt the 25g/1oz butter with the olive oil in a large frying pan. Add the chicken schnitzels and cook for about 2-3 minutes on each side until golden and cooked through.
To serve, pile the mash onto one third of the plate with a little spinach to the side, lay the schnitzel on a remaining third and finish with a little sauce.