Lightly grease a 10-inch quiche/tart pan (or a pie plate).
In a medium frying pan, cook diced onion with a bit of vegetable oil (or cooking spray) over medium-high heat until translucent and tender. Add in fresh spinach and cook until just wilted. Set aside to cool for a few minutes.
In a large mixing bowl, whisk together eggs, flour, and baking powder. Whisk in milk, then stir in spinach-onion mixture.
Pour quiche base into prepared pan. Top with feta cheese.
Bake for 25 minutes, or until center is set and the outside edge is golden brown.