Portion the fish (approx. 250 g each), season with salt and place in a baking dish. Quarter the tomatoes, slice the onions, squeeze the lemon. Put everything in the baking dish along with a cup of water.
Preheat the oven to 200 ° C. Put the casserole dish in the hot oven with the lid closed and cook for 20 minutes.
Then pour off the brew through a kitchen sieve and collect it. Melt the butter in a small saucepan and use the flour to create a roux. Then deglaze with the collected south and apple juice (to taste), season with salt.
Serve the fish in the baking dish, add the apple sauce in a sauce bowl.