Peel, rinse and slice the potatoes. Rinse the cauliflower and cut into florets. Rinse the leeks and cut diagonally into 0.5 cm slices (approximately 1/5 inch). Cut the bacon into strips.
Peel and dice the onion. Layer the potatoes, leeks and cauliflower into a greased baking dish. Sweat the onions in hot butter, then mix with the creme fraiche, milk, mustard, salt and pepper. Stir in the eggs and half of the cheese, then pour over the vegetables. Sprinkle the top with the remaining cheese. Cook in a preheated oven at 180°C (approximately 350°F) for 45 minutes. Meanwhile, fry the bacon until crispy and drain into a small pan.
Serve the finished casserole garnished with bacon.