Pat the tilapia fillets dry with a kitchen towel. Cut into two-inch cubes. Toss lightly with salt and pepper.
Slice the mushrooms.
Peel and roughly chop the onion.
Heat the butter in a pan. Add the chopped onion and cook gently over medium heat until translucent, about 10 minutes.
Turn up the heat to high. Add the fish to the pan and cook, stirring gently, for about a minute.
Add the mushrooms. Pour in the fish stock. Bring to the boil. Simmer for a minute.
Pour in the cream. Stir gently. Season with salt and pepper. As soon as the sauce boils, turn off the heat. The residual heat will cook the fish and mushrooms through.
Transfer the tilapia and baby portobello mushrooms in cream sauce to a shallow bowl, garnish with parsley and serve at once.