Place the feta on a paper towel and pat dry on all sides, absorbing any excess liquid. Transfer the feta to a cutting board and use a large chef’s knife to cut the feta into small dice, about 1/2 inch (12 mm) in size. 

Place the diced feta in a shallow dish that is big enough to hold the cheese in a single layer. Sprinkle the jalapeño slices over the feta. In a small bowl, stir together the remaining ingredients except for the sumac and pour over the feta cheese. Cover the dish and marinate for 30 minutes unrefrigerated, or for up to one week in the fridge.

Garnish with sumac before serving.

Make-Ahead Tip: Marinated feta can be stored in the fridge for up to 1 week. The oil in the feta may solidify while refrigerated, so simply place the dish on the counter for 20 minutes before serving to allow the oil to liquefy.