This Keto Fish Casserole with Mushrooms and French Mustard is a delicious and healthy low-carb dish. The fish is baked with a creamy mushroom sauce and tangy French mustard, creating a flavorful combination. It's a perfect option for those following a ketogenic diet or looking for a nutritious meal. The casserole is easy to make and can be enjoyed as a main course with a side of vegetables or salad.