In a small saucepan, combine the brown sugar and water. Heat gently over medium heat, stirring just until the sugar fully dissolves. Do not boil.
Remove from heat and add the grapefruit peel strips and the pinch of fine sea salt. Let steep for 6–8 minutes, just until aromatic and lightly bitter. Remove and discard the peel. Let the syrup cool completely.
Spoon the grapefruit pulp into a bowl, removing any seeds. Lightly mash with a fork just a few times to loosen the juice while keeping pieces intact. Do not blend — texture is intentional.
In a pitcher, combine the mashed grapefruit pulp and 80–100 ml of the cooled grapefruit peel syrup. Stir gently to combine. Taste — it should be bitter, lightly sweet, and textured.
Add plenty of ice to the pitcher. Pour in the chilled tonic water and stir gently once or twice to integrate without killing carbonation.
Pour into ice-filled glasses and garnish with a twist of pink grapefruit peel.
