Start by pitting each Castelvetrano olive if not already pitted, creating a small cavity for the filling. Using a small spoon or your fingers, carefully stuff about 1/4 teaspoon of feta cheese into each olive. Arrange the stuffed olives on a plate as you work.

In a medium glass jar or bowl, combine 1/3 cup of olive oil, 2 teaspoons of lemon zest, and the grated meat of 2 garlic cloves. Add 1 teaspoon of red pepper flakes, 1/2 teaspoon of sumac (if using), and a pinch of salt. Whisk or shake until well blended.

Carefully place the stuffed olives into the prepared marinade, gently tossing to ensure all are well coated. Let them soak in the flavors for at least 10 minutes at room temperature.

After marinating, gently stir the olives to redistribute the marinade. Allow them to come to room temperature for at least 10 minutes before serving.

Transfer the olives onto a serving platter, drizzling some of the extra marinade over the top to entice your guests.