Juice your lemons: Start by rolling them firmly with your hand on a hard surface to loosen the juice. Cut them in half and juice using a juicer. Set aside.
Add the pineapple chunks and the pineapple juice to a high-speed blender and blend until smooth. Strain the mixture with a fine mesh strainer, saving the liquid and discarding the pulp.
Pour the pineapple liquid, fresh lemon juice, 3 cups of water, and the simple syrup into a serving pitcher and stir until well combined.
Pour over ice and garnish with fresh pineapple slices and/or mint leaves, and serve!