Line an 8x4-inch (or similar) loaf pan with parchment paper, leaving an overhang for easy removal; lightly oil the paper or pan.
Roughly chop and, if you like, lightly toast the Pistachios in a dry skillet until fragrant; set aside to cool.
In a medium saucepan combine the Granulated Sugar, Water and Lemon Juice. Stir over medium heat until the sugar dissolves.
Bring the syrup to a gentle boil without stirring; continue boiling until it reaches the soft-ball stage (about 235–240°F / 112–116°C) or when a small amount dropped into cold water forms a soft, pliable ball. Remove from heat.
Let the syrup rest for 2–3 minutes, then stir in the Vanilla Extract.
Place the Tahini (sesame paste) in a large, heatproof bowl. Pour the hot syrup over the tahini in a thin, steady stream while whisking or stirring vigorously. Add the syrup gradually and continue to mix until the mixture thickens and starts to become crumbly and matte in appearance.
Fold in the toasted pistachios and a pinch of Fine Salt to taste, reserving a few nuts for the top if desired.
Transfer the halva mixture to the prepared pan, press it firmly and evenly with a spatula or the back of a spoon (use parchment to help smooth the surface). Sprinkle remaining pistachios on top and press lightly.
Allow the halva to cool to room temperature, then refrigerate for at least 2–3 hours (or overnight) until firm. Use the parchment overhang to lift the block from the pan, slice thinly, and serve. Store in an airtight container at room temperature or refrigerated for longer freshness.
