Cut fish into 1/2? thick slices (if it’s not already). Sprinkle salt and fresh ground black pepper liberally all over the fish, and set aside.
Melt butter in a Dutch oven over medium heat
Add the onions and saute for a few minutes, until they become translucent. Be sure not to brown them in any way.
Reduce heat to very low, cover the Dutch oven tightly, and cook onions for about 20 minutes, stirring once or twice to prevent browning. Onions should be fully flavored, lightly yellow, but not brown, and most of the onion juices should be gone. If this state isn’t reached in 20 minutes, cover and cook longer, checking every 5 minutes for doneness.
Whisk the eggs and cream together
Remove onions from heat, add egg and cream mixture and stir very thoroughly, until creamy and uniform.
Season with salt and pepper to taste, add nutmeg.
Finally, add the cheese and stir gently to distribute.
Add paprika if desire.
Sprinkle the surface of the table with flour lightly
Roll out puff pastry to be slightly larger than the baking pan (I used a 10? pie pan).
Line the baking pan with the layer of puff pastry, press it into the sides of the pan to form a pie shell.
Pierce the shell all over with the fork
Fold out half of the egg and onion mixture into the baking pan
Arrange the fish slices over in a single layer, covering the entire surface of the pie
Fold out the other half of the egg and onion mixture over the fish layer
If you wish to make patterns in the pie, make them at this time. I improvised and used a plastic cap of a water bottle to cut out incomplete circles in the crust. While baking, the circles puffed out and shrunk, creating little peek-a-boo openings in the pie.
Carefully transfer the top pie crust onto the top of the pie. Stretch out if necessary to distribute your patterns nicely.
Using a rolling pin, press the pie crusts together to the rim of the baking pan, thus cutting the extras off without stretching the crusts.
Bake for about 30 minutes at 425°F, or until the egg and onion mixture is set in the middle, and pie crust is golden and flaky.