Preheat oven to 350ºF. You will need an 8 or 9-inch tart pan with a removable bottom or a pie plate.
First make the lemon cream: Place the eggs, yolks and sugar in a heatproof bowl and set over a medium saucepan with simmering water (or use a double-boiler). Add the lemon juice and zest and whisk. Cook the lemon cream uncovered whisking occasionally, until thick. This takes approximately 30 – 35 minutes. Be patient and do not stir too much. If the water in the saucepan boils too fast, turn down the heat.
Remove the bowl from the heat and whisk in the butter or margarine a tablespoon at a time. Set aside.
While the lemon cream is cooking, prepare the crust: Melt the butter or margarine either in the microwave oven for 45 seconds, or on the stovetop, until melted. Add the walnuts, almonds and sugar and mix until combined. Add the chopped basil and mix in. Scoop into the pie pan and use your hands to press to cover the bottom and go up the sides. Try to even out the rim at the top. Place in the oven 15 minutes.
Remove from oven and spoon the lemon filling into the crust. Bake for 20 minutes, or until the outside edges of the cream are set; the inside can remain a little wobbly. Let cool and then place in the fridge for at least two hours or overnight.
To make the meringue topping: In a small heavy saucepan, bring the sugar and water to a boil, stirring to dissolve the sugar. Continue to cook the sugar until it reaches 230ºF on a candy thermometer. While the sugar is cooking, beat the egg whites in a medium bowl with an electric mixer on high speed until stiff. When the sugar is ready, turn the mixer speed to low and then slowly pour in the cooked sugar down the side of the bowl, not directly onto the wire whisk. When all of the sugar has been poured in, turn the mixer up to high for one minute.
Use a silicone spatula to spread the meringue all over the top of the lemon or use a pastry bag and tip to make any decorative designs. You can use a blowtorch to lightly brown the meringue or place the tart in a 450ºF oven for a few minutes, watching the entire time, until the top browns. Store in the fridge for up to three days.