For the spinach mixture
Heat the oil in a large skillet, add the spinach and onions and cook for a few minutes until tender. Remove the mixture from the heat, mix in the remaining ingredients and set aside.
For the batter
Place the flour, salt, pepper and sugar in a bowl and stir well. Make a small hole in the middle and pour in the eggs, vinegar and beer. Whisk ingredients together until blended.
For the tempura
Heat olive oil in a deep frying pan. Using a fork, take a portion of the spinach mixture, dip it into the batter and then place it into the hot oil. Repeat with the remaining spinach mixture. Fry for 1-2 minutes on each side until golden brown. Remove from hot oil and place on paper towels to soak up any excess oil.
Transfer to serving platter and sprinkle with crumbled feta cheese. Serve with lemon wedges.