Prepare kabocha: Remove seeds, cut, and steam until soft (about 10 mins).
Peel kabocha, and place in a bowl.
Chop onion finely, and cook until translucent. Mix with kabocha.
Make bechamel sauce: Melt butter, and whisk in flour until bubbly.
Pour in hot milk, whisk, add salt & pepper. Thicken.
Add bechamel to the kabocha mix. Blend until creamy. Add seasoning.
Transfer to a casserole dish. Place herring on top.
Add shredded white cheese. Cover with puff pastry.
Cut fish design from another pastry sheet. Place on top.
Apply egg wash. Put cut olives around the dish.
Bake at 375°F for 25-30 minutes.
Enjoy immediately.