Preheat your oven to 170°C.
Put the flour in a bowl.
Place the eggs in another bowl and whisk.
In a large saucepan put the water, butter, sunflower oil, salt and caster sugar and bring to the boil. Remove from the heat and stir in the flour, mix until well combined.
Place back on medium heat and cook for around 2 minutes, stirring all the time. You'll notice how the mixture begins to come away from the side of the pan.
Remove pan from the heat and allow to cool for a couple of minutes.
Whisk in the eggs a little at a time until the mixture reaches a thick dropping consistency, you may need to add the extra egg. The mixture is ready when it makes a v-shaped dropping off the spoon.
Place mixture in a large disposable piping bag, ideally fitted with a 15mm star nozzle and place on one side to cool slightly before piping.
Take a baking sheet and on a piece of baking parchment mark twelve 15cm lines, then turn over so you can see the lines through the parchment.
Dab some spots of the choux pastry mix on the baking tray and stick the parchment to it. Pipe 12 eclairs using the lines you've drawn as a guide. Bake in the centre of the preheated oven for 35 minutes until well risen, golden and crisp.
Remove from oven and allow to cool.
Put the blueberries in a small food processor and puree.
Place the blueberry puree in a large saucepan with the lemon juice and jam sugar and bring to the boil stirring all the time. Add the knob of butter and boil for 4 minutes.
Pour into a heatproof bowl and leave on one side to cool.
In a large bowl whisk together the double cream and icing sugar until it begins to thicken. Do not over whip or it'll be tricky to pipe.
Cut each eclair neatly in half then pipe a line of blueberry jam on the base of each eclair.
Now top the jam with piped cream.
Take the fresh blueberries and place a row of them in each base on top of the filling the top with the remaining eclair half.
Add a little water to the fondant icing sugar until you get a thick paste.
Put in a piping bag ideally fitted with a 15mm flat nozzle.
Pipe a line of fondant over the top of each éclair and decorate with blueberries and walnuts. Serve.
You can freeze the eclairs unfilled.