Peel and slice the potatoes. Place half in a greased ovenproof dish and season well. Sprinkle with the onion, tarragon and bacon then cover with the rest of the potatoes. Pour over the cream/milk and cover with greased foil.
Cook in the oven (200C) for about 1 and a half hours or until almost tender. Remove foil and sprinkle on cheese. Return to oven, uncovered for 15-20mins.