Cut the fillet into 4 oblong slices. Slice carrots and asparagus lengthwise, halve green onion lengthwise.
Sauté carrots and asparagus in butter for 3 minutes, remaining vegetables for another 5 minutes.
Mix crème fraîche and mustard.
Cook lasagna sheets until al dente, dry and place in a buttered ovenproof dish so that they rise over the edge.
Place fish on top of a salted and peppered layer of vegetables and cover with vegetables, spreading 2 tablespoons of mustard cream on top.
Fold the sheets over and cook at 225 °C on the 3rd rack from the bottom for 10 minutes. If necessary, brown briefly under grill function at the end.