Rinse and pat dry the cod fillet. Season both sides with salt and pepper.
Finely chop the onion and cook in butter over medium heat for 3-4 minutes until softened.
Spread the cooked onions in a baking dish. Place the cod fillet on top. Spread the frozen peas over the cod. Halve the bananas lengthways and arrange on top of the peas.
In a bowl, mix the cream, crème fraîche, curry powder, sugar, and lemon juice until combined.
Spread this mixture evenly over all ingredients in the baking dish.
Bake at 170°C / 338°F using convection on the middle rack for about 50 minutes until the cod is cooked through and flakes easily.