After cleaning the medium-sized trout, we scratch it and cut off its head (no need if you can fit it in the pot).
We salt it, place it in the casserole dish, add some of the onions we cut into half rings, and arrange all the remaining ingredients around the fish.
We salt it again and drizzle it with olive oil. We foil it, make a few air holes, and put it in the oven that we preheated to 200 degrees.
After half an hour, we open the foil and let it cook again.
For the last 10 minutes, we put the butter into the casserole and wait for the cooking time to complete.
It is delicious with water and butter. 1 casserole per person is recommended.