In a bowl, mix the sour cream, breadcrumbs, and gratin cheese with salt to taste until you have a thick, spreadable mixture. Set aside.
Heat a splash of oil in a small saucepan over medium heat. Add the chopped onion and crushed garlic and cook for 3-4 minutes until softened.
Stir in the canned tomatoes and tomato paste. Add 1 teaspoon sugar, 1/2 teaspoon salt, a pinch of pepper, and the mixed herbs. Simmer for 10 minutes, stirring occasionally. Set aside.
Wash the thawed fish fillets under cold water and pat dry with paper towels. Drizzle with lemon juice.
Preheat the oven to 190°C (375°F). Grease a large baking dish with butter and arrange the fish fillets in a single layer. Season with salt and pepper to taste.
Spread the tomato sauce evenly over the fish fillets. Top with the cheese mixture in an even layer.
Bake for 45 minutes until the top is golden. Serve with rice, potatoes, pasta, or baguette.