Clean, wash and cut the leek into rings. Blanch in boiling salted water for eight minutes and drain. In the meantime, drain the pineapple. Fry in a tablespoon of hot fat for five minutes on each side.
Put the leek and pineapple with halved bacon in an ovenproof dish. For the crust, peel and coarsely grate the potatoes. Wash the thyme and pluck the leaves from the stems. Grate cheese. Mix everything. Season with salt and cayenne pepper.
Spread on the casserole. Bake in the preheated oven (electric oven: 200 °C/ gas: level 3) for 20 minutes. In the meantime, peel and finely dice the onions for the sauce. Wash and chop the parsley. Heat the remaining fat.
Brown the onions in it. Sauté the flour in it. Deglaze with broth and cream while stirring and bring to the boil. Stir in parsley. Season to taste with salt, pepper and mustard. Serve the casserole garnished with fresh herbs and the sauce.