Combine the sliced strawberries, 1.5 cups of sugar, and 2 cups of water in a medium saucepan. Place the pan over medium heat and let it gently simmer for 5 to 10 minutes. Stir occasionally until the sugar dissolves and the strawberries become soft.

Set a fine-mesh strainer over a large bowl or pitcher. Pour the warm syrup through the strainer, pressing the berries with a spoon to extract all the liquid. Discard the solids left in the strainer.

Squeeze the lemons until you have 1.5 cups of fresh lemon juice. Strain the juice if you prefer a smoother texture without pulp.

In a large pitcher, combine the cooled strawberry syrup and fresh lemon juice. Add the remaining 5 cups of cold water and stir well. Refrigerate for 1 to 2 hours to allow the flavors to mingle, then serve over ice.