Line a loaf pan with parchment paper with a little overhang for easier lifting. Set aside.
In a small bowl, combine tahini, rose water, and salt. Set aside.
In a small saucepan over medium-high heat, add the sugar and slowly pour the water to cover the sugar. Cook, until mixture reaches 235°F on a candy thermometer, 10-15 minutes.
Carefully and slowly pour the tahini mixture into the syrup mixture and stir until fully incorporated and glossy. It will thicken as you stir. Fold in the pistachios.
Pour into the prepared loaf pan. Sprinkle more pistachios on top and smooth the top. Allow it to cool at room temperature for 30 minutes. Then cover with plastic wrap and refrigerate until set, about 2 hours. Slice into 8 or 10 slices and serve.
Notes
Storage: I usually store halva in the fridge! If it gets too warm at room temperature, it can get oily and soft. Kept in the fridge in an airtight container, it'll keep for about a week.