Preheat the oven to 180C/160C Fan/Gas 4. Have ready a 4cm/1½in deep, 20cm/8in pastry ring or cake tin.
For the oat biscuit base, in a large bowl combine the flours, butter, bicarbonate of soda, sugar and salt. Using your fingertips, rub the ingredients together until the mixture resembles breadcrumbs. Add the oats and egg and mix well. Knead until it comes together.
Using a rolling pin, roll the biscuit mixture out between two sheets of baking parchment to a thickness of 5mm/¼in. Lift onto a baking tray and remove the top piece of paper. Bake in the oven for 15 minutes.
Remove from the oven and leave to rest for 2 minutes. Using the 20cm/8in pastry ring or cake tin, press down into the biscuit, cutting through the bottom. Leave to cool completely before using for the cheesecake. Remove the excess biscuit from around the ring; this can be crumbled on top of the finished cheesecake, if wished.
For the filling, in a large bowl mix the cream cheese, crème fraîche, vanilla purée or extract and lemon juice together.
Place the gelatine in a small saucepan with 2 teaspoons water and heat gently until the gelatine is dissolved. Once slightly cool, briskly whisk into the cream mixture. In a small saucepan dissolve the sugar in 5 teaspoons water. Bring to a boil and continue to heat to 121C on a sugar thermometer. (CAUTION: boiling sugar is very hot. Take care not to burn yourself.)
Meanwhile, in a food mixer fitted with the whisk attachment or using an electric hand whisk and a bowl, whisk the egg whites, adding just a single drop of lemon juice at the beginning. Whisk until soft peaks form when the whisk is removed.
Turn the whisk to a medium speed and prepare to add the hot sugar syrup. The best technique for making this kind of Italian meringue is to pour the hot sugar syrup down the side of the bowl in a single thread so that it mixes beautifully. Otherwise the speed of the whisk will spin the sugar and solidify it over the whisk and bowl. Continue to whisk on a medium speed for a further 5 minutes until the mixture is glossy – the hot syrup will partially cook the egg white making it stable.
Rest the base of the bowl of meringue in a bowl of ice-cold water and continue to whisk by hand until the mixture is completely cool. When cool, add half the egg whites to the cream filling mixture and briskly beat with a whisk until smooth, then add the remaining egg whites and fold together.
Place the pastry ring or cake tin with the biscuit base inside on a baking tray or plate. Pour the filling into the ring, leaving a gap of 5mm/¼in from the top of the ring and level the surface flat using a palette knife or the back of a spoon. Refrigerate for 2 hours to allow the mixture to firm up.
When the cheesecake has chilled for 1½ hours, make the gooseberry jelly. Put the gooseberries in a bowl and macerate with the sugar for 15 minutes. Transfer to a small saucepan over a medium heat and simmer for 10 minutes, covered with a lid. Pour into a blender, with the softened gelatine and 50g/1¾oz water and blend on full speed for a minute. To make sure the purée is smooth, pass it through a fine sieve into a bowl, forcing it through using the back of a ladle. Rest the base of the bowl over a bowl of ice-cold water to cool, stirring all the time. Once the mixture starts to thicken, spoon it over the chilled cheesecake.
For the topping, top and tail the gooseberries and cut each one crossways into slices 2mm thick. Arrange 16 slices of gooseberries around the edge of the cheesecake on top of the jelly. Chill the cheesecake overnight or until set completely.
For the compôte, cut the gooseberries in half. Taste one – if sweet enough, there is no need to macerate them, if they are a little sour, put in a mixing bowl with the sugar and leave to macerate for 15 minutes.
Transfer to a saucepan on a medium heat and cook for 10 minutes with a lid on, stewing them at a gentle bubble; stir from time to time to prevent the compôte catching on the bottom of the pan. Remove from the heat and allow to cool completely, taste and adjust with a splash of water if it’s too acidic or a little sugar, if needed.
Serve the cheesecake with the compôte alongside for your guests to help themselves.