Preheat the oven to 140°C/120°fan/Gas 1. Line a baking sheet with baking paper.
Whisk the egg whites until stiff, then gradually whisk in the sugar, a tablespoon at a time, and continue whisking until the mixture is very thick. Spoon the mixture onto the baking paper and spread it out to a circle 20–25cm (8–10in) in diameter, hollowing it slightly in the centre.
Bake for 1¼ hours, then turn off the oven and leave the meringue in the oven to cool.
Meanwhile, make the toffee sauce: melt the butter in a saucepan, stir in the sugar and then the crème fraîche and stir over a low heat until the sugar has dissolved. Increase the heat and simmer for 5–6 minutes until slightly thickened. Remove from the heat, stir in the vanilla and salt and leave the sauce to cool completely, stirring occasionally.
Whip the cream with the icing sugar until soft peaks form. Slice the bananas and toss with the lemon juice. Put the meringue on a serving plate and fill with the cream. Top with the bananas and drizzle with sauce.
Instead of bananas you could use chopped pears softened in a little butter.