In the bowl of a blender or food processor, add the feta cheese and sour cream/yogurt. Blend on medium speed continuously until mixture is creamy and lump free (about 1 minute).
Drizzle in olive oil while blender is still running, until combined. Add the lemon zest and black pepper and pulse to mix. Taste, and if you need it, add salt.
Transfer the dip to the small bowl you'll serve it in. Make your garlic chips if using (see notes below)
For the garlic chips (optional) and garnish:
Heat the olive oil in a small skillet over medium heat until hot. Add sliced garlic and fry on each side for 1-2 minutets until golden brown, then immediately drain the oil and place garlic on a paper towel to remove excess oil. Let garlic chips cool.
Just before serving, top the dip with a drizzle of olive oil, a pinch of red chili flakes, garlic chips, and chopped pistachios.
If not using garlic chips, add 1 crushed garlic clove to the dip while blending. If not serving dip straight away, it'll keep up to 3 days sealed in the fridge. Garnish just before serving.