For the leeks and beans, in a shallow pan, gently fry the leeks in the butter and oil, adding a splash of water and the vegetable bouillon powder when they start to sizzle. Cover the pan with a lid and, when the leeks are nearly cooked (about 10 minutes), stir in the flageolet beans. Put the lid back on the pan and cook for another couple of minutes, giving the pan a good shake every now and then, until the beans are heated through and the Ieeks are tender.
Meanwhile, for the pork, season the pork with salt and freshly ground black pepper. Heat the butter and oil in a large frying pan over a medium-high heat and, when the butter is foaming, add the medallions to the pan and brown for 2-3 minutes on each side (you may need to do this in batches to avoid overcrowding the pan). Transfer each batch of pork to a warm plate.
Deglaze the pan by pouring in the Calvados followed by the apple juice and scraping up the burned bits from the bottom of the pan using a wooden spoon. Bring the mixture to the boil, let it bubble for a minute or so to reduce slightly, then stir in the shredded sage, mustard, garlic and cream. Return the pork medallions to the pan, along with any juices that have collected on the plate, and continue to cook in the sauce for another 2-3 minutes, or until the pork is cooked through.
To serve, place a pile of leeks and beans onto each plate, arrange the pork medallions on top and spoon over the sauce. Garnish with the whole sage leaves.