White Fish Gratin

Ingredients for fish  mixture:

• 1- 1 1/2 kg fish , fillet (Nile perch, boneless)
• 1/2 kg button mushrooms, cut in quarters
• 1 onion, chopped
• 1 carrot, cut into small cubes
• 2 Tbs oil
• 1/2 C milk, for marinade

Ingredients for bechamel sauce (white sauce):

• 750 mg milk  
• 100 mg 15% cream (optional)
• 40 gr flour
• 40 gr butter
• 1 tsp mustard
• 1 tsp salt
• 1/4 tsp  nutmeg
• a pinch of pepper
• 150 gr gruyere (Swiss cheese for the top)

Directions for fish mixture:

1. Place  fish in a shallow bowl and marinate with milk for at least an hour.
2. Drain fish well .
3. Boil enough water to cover fish in a large pan. Add salt and pepper.
4. Add fish. Cook for a few minutes until fish  has changed color and is white. Don't over cook.
5. Drain and cut fish into small pieces. Transfer fish to a large bowl.
6. In a large frying pan put  oil and cook  onions, then add carrots. Simmer for a minute. Add mushrooms. Cook until all liquid is evaporated.
7. Combine delicately fish and mushroom mixture.

Directions for  Bechamel Sauce

1. Melt butter in a medium saucepan over medium heat.
2. Add flour
3. Cook stirring for 1 minute or until bubbly.
4. Remove from heat
5. Slowly add milk, whisking constantly until mixture is smooth.
6. Add salt, pepper and nutmeg.
7. Return to heat. Cook stirring with a wooden spoon until sauce is thick.  
8. Stir in cream (optional)  and continue cooking for a few minutes.
9. Remove from heat. Stir in mustard.

Directions

1. Stir in bechamel  sauce delicately to fish mixture.
2. Prepare 6-8 shallow baking dishes side by side.
3. Pour  fish- mushroom -bechamel mixture equally in  small baking dishes.
4. Spread grated gruyere on top.
5. Bake on 200 C preheated oven until  golden brown.
6. Serve hot.