Pat the tilapia fillets dry with a kitchen towel. Cut into two-inch cubes. Toss lightly with salt, pepper and lemon zest.
Slice the mushrooms.
Peel and roughly chop the onion.
Heat the butter in a pan. Add the chopped onion and cook gently over medium-low heat until translucent.
Turn up the heat to medium-high. Add the fish to the pan and cook, stirring gently, for about a minute, or just until opaque.
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.
Cut the lemon into halves or quarters and squeeze directly over the fish and mushrooms. Stir gently. Taste and adjust the seasonings, as needed.