For the filling
For the omelette
To make the filling, heat the butter in a frying pan over a medium heat until melted. Add the mushrooms and sauté for 2 minutes. Add the courgette, and continue to sauté for 2 minutes.
Add the tomato and cook over a high heat for 2 minutes to evaporate any excess liquid from the tomato. Add the salt, pepper and basil, and mix well. Taste for seasoning. Cover to keep warm.
To prepare each omelette, whisk 2 eggs with a pinch of salt, a pinch of pepper and 1⁄2 teaspoon club soda until smooth.
Melt 15g/1⁄2oz of the butter in a frying pan over a medium heat until it begins to sizzle.
Pour in the egg mixture, and stir it in the centre of the pan with the flat side of a fork. Using the prongs of the fork, lift the edges of the omelette so that any uncooked mixture runs to the edge of the pan. Vigorously slide the pan back and forth over the heat until the omelette begins to slip around freely.
When the omelette is lightly cooked but still creamy in the centre, spoon about 2 tablespoons of the filling over the half of the omelette closer to the pan’s handle. Sprinkle 1 tablespoon of the Parmesan over the filling.
Fold the omelette in half, and slide the folded omelette onto a serving dish. Serve immediately. Repeat with the remaining eggs.