1. To toast the pine nuts, adjust the oven rack to the middle position and preheat the oven to 160 °C.
2. Scatter pine nuts on a baking sheet and place it in the oven to toast, shaking the pan occasionally, for 8 to 10 minutes, or until they are fragrant and golden brown.
3. Combine the pine nuts, garlic, salt and half of the olive oil in the bowl of a food processor fitted with a metal blade, or in the jar of a blender.
4. Add the remaining basil, the Parmigiano-Reggiano and the remaining olive oil and purée, stopping as soon as the ingredients form a homogenous paste and adding more olive oil if necessary to obtain a loose, spoonable pesto.
5. Turn the pesto out into a bowl and stir in the lemon juice.
6. Use the pesto or transfer it to an airtight container and refrigerate for up to 2 day; any longer and it will lose its pretty green colour and vibrant flavour.