In a heavy bottom pot, melt the butter and add the shallot oil. Add the flour and mix well with the butter. Add the milk and stir constantly until thickened. Add the cheese. Set aside. Peel the potatoes. Slice the potatoes as thinly as possible with a mandoline. Cut the root and tough green level from the leek. Slice the leek the same. Butter a casserole and put half the potatoes in. Top with half the leeks, half the sauce, then the remaining potatoes, and remaining leeks. Dust with paprika. Pour the remaining sauce mixture over the potatoes, then top with breadcrumbs. Bake covered at 400 for 1 hour, removing the top for the last 15 minutes.