Prepare the pear purée: Place chopped pears in a blender with 1 cup of water. Blend until completely smooth, about 45-60 seconds. Strain through a fine-mesh sieve into a large bowl, pressing with a spoon to extract maximum juice. Discard solids or save for another use.
Create the basil syrup: In a small saucepan, combine honey/agave with 1 cup of water and bring to a gentle simmer over medium heat. Once simmering, remove from heat, add Thai basil leaves, and stir to submerge. Cover and let steep for 8-10 minutes until the syrup is fragrant and has taken on a slight green tint.
Strain and cool the basil syrup: Remove basil leaves (they’ll have turned darker) and allow the syrup to cool to room temperature. This infusion can be made ahead and refrigerated for up to 3 days.
Combine the base ingredients: In a large pitcher, mix the strained pear purée, freshly squeezed lemon juice, and basil-infused syrup. Add the salt and stir thoroughly to combine.
Dilute and adjust: Add the remaining 4 cups of cold water and stir well. Taste and adjust sweetness by adding more honey/agave if desired, or more lemon juice for extra tartness. Remember that over ice, flavors will become slightly diluted.
Chill thoroughly: Refrigerate for at least 1 hour before serving to allow flavors to meld. If time is short, add extra ice when serving.
Serve with style: Pour over ice in tall glasses, garnish with fresh Thai basil sprigs and lemon slices. For an extra visual appeal, add a few thin pear slices to each glass.