Heat a grill pan over medium-high. Add pineapple slices in a single layer, and cook 5 to 6 minutes on each side, until dark char marks appear. Set grilled pineapple slices aside to cool.
Combine 4 cups hot water and sugar in a large pitcher, and stir with a whisk until sugar is dissolved.
Process pineapple and lemon juice in a blender until relatively smooth.
Pour pineapple mixture through a fine-mesh strainer into pitcher with sugar mixture; discard solids. Stir well until fully blended. Serve over ice.
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