Put the spinach in a dry frying pan on high heat for a couple of minutes, stirring constantly. Take off the heat as soon as the leaves start to wilt. Let the leaves cool on a kitchen towel to remove any excess liquid. When cool, pat dry, chop coarsely, and season to taste.
Mix all the ingredients for the burgers in a bowl. Season with freshly ground black pepper and just a little salt because of the feta. Shape into 8 burgers. Chill in the fridge for 15 minutes
Pre-heat the grill to a high setting. Transfer the burgers onto an oven or baking tray lined with parchment paper. Brush the burgers with olive oil and grill for around 10 minutes on each side until cooked through and golden brown on both sides.