1. Heat the oil in a sauté pan or cast iron skillet over medium heat. Add the onions and cook until tender and translucent, about 5-6 minutes.
Add 1 ½ tablespoons of lemon juice to the pan, followed by the spinach leaves. Toss the leaves in the pan 1-2 minutes, or until the leaves are beginning to wilt and turn darker green (be careful not to overcook at this stage or your pinch will turn to mush).
Remove from the heat and add a generous amount of salt and pepper. Sprinkle the spinach with the feta and 1 teaspoon of lemon zest, tossing gently. Serve promptly while hot.