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To make the praline, put the sugar into a heavy-based pan with 3 tbsp water. Place over a low heat until the sugar has dissolved and is clear. Don't stir, or it will crystallise. Increase the heat and bubble to a dark caramel (see video technique), about 5 mins. Remove from the heat, stir through the nuts, then tip onto a lightly greased non-stick baking tray. Spread to level and leave to cool. Be careful as the tray will get very hot.
When the praline is cold and brittle, break it into pieces. Then, using the end of a rolling pin, smash it up until fairly well crushed. If this is proving hard work, tip into a food processor and pulse until it’s the texture of coarse breadcrumbs.
Line a 1.2-litre loaf tin with a strip of baking parchment (this size tin gives a neat shape, but you can set the parfait in any shaped tin, or individual pudding basins or ramekins). In a bowl, mash the bananas with half the lemon juice until you have a rough purée, then set aside.
Put the cream into a bowl and whip until it holds its shape but is still a little soft. In another very clean bowl and using a very clean electric whisk, beat the egg whites with a squeeze more lemon juice until stiff. Slowly add the sugar until you have a stiff, shiny meringue.
Gently fold the whipped cream and meringue together, then add the banana and most of the praline, leaving about 4 tbsp to serve. Spoon into the loaf tin, smooth the top and freeze until firm, preferably overnight.
To serve, remove the parfait from the freezer 10 mins before serving and leave in the fridge to soften slightly. Meanwhile, slice the bananas on a slant, allowing 3 slices for each serving. Lay the slices on a baking tray set on a wooden board and scatter heavily with the sugar. Use a blowtorch to caramelise the banana, then set aside. Be very careful as the tray will be extremely hot. If you don't have a blowtorch, place the slices into a hot, dry frying pan and sear the sugar-coated sides until caramelised. Lift and flip them over to a tray.
To plate up, sprinkle a neat line of praline just off-centre on each plate. Arrange the slices of caramelised banana at opposing angles. Unmould the parfait, then use a warmed knife to neaten the sides and cut into finger-thick slices. Lay the slice on the opposite side of the plate to the banana and serve straight away.