Creamy mornay fish hot pot with dill potato gratin

Ingredients

• 40 grams butter
• 1 onion, diced
• 1 stick celery, diced
• 1 small carrot, grated
• 1/2 teaspoon whole grain mustard
• 1/4 cup plain flour
• 1/4 cup white wine
• 1 1/2 cup hot milk
• 500 grams diced white fish
• 1/4 cup grated Parmesan cheese, plus extra for topping
• 1/2 cup grated gruyère cheese
• 2 tablespoons butter, melted
• 2 medium Agria potatoes, very finely sliced
• fresh dill
• salt to taste