In a medium saucepan, combine frozen mixed berries, granulated sugar, and water. Bring to a simmer over medium heat, stirring occasionally, and let simmer for 20 minutes.

Remove the saucepan from heat and let cool for 20 minutes. Strain through a fine-mesh sieve into a bowl, pressing the berries to extract maximum juice.

Wash and halve the lemons. Squeeze to extract juice until you reach 1 cup.

In a large pitcher, mix the strained berry syrup with the prepared lemon juice and cold water. Stir well and adjust sweetness if needed.

Fill highball glasses with ice and pour the Berry Lemonade, filling each glass about three-quarters full.

Garnish with fresh lemon slices and extra berries.