Toss 1 strawberry and 4 basil leaves into two large highball glasses. Muddle the berries with the basil leaves, crushing them together.
In a cocktail shaker add the gin along with the lemonade and the Cointreau.
Fill the shaker with ice and shake until very cold, about 1 minute.
Fill each highball glass with ice and divide the cocktail between them. Pour ¼ cup lemon soda in each glass to top off the cocktail. Stir to blend well.
Garnish with a strawberry slice and a slice of lemon.
For the fresh lemonade:
Pour ½ cup of the water into a small saucepan. Add the sugar and warm over medium heat. Stir frequently until the sugar has completely dissolved, about 5 to 7 minutes.
Let the lemon simple syrup cool for about 10 minutes then pour it into a pitcher. Add the lemon juice.
Pour in the remaining 3 cups of water and stir to mix well. You can add more lemon juice or water if desired to suit your taste.