Pre-heat the oven to 400°F.
Lightly grease a 10-inch quiche/tart pan (or a pie plate).
In a medium skillet, over medium-high heat, cook diced onion with a bit of vegetable oil or cooking spray until translucent and tender.
Add the fresh baby 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 and then stir in the spinach-onion mixture.
Pour quiche base into the prepared pan; top with feta cheese.
Place in the pre-heated oven and bake for 25 minutes, or until center is set and the outside edge is golden brown.
Remove from the oven and allow the quiche to set for 5 minutes; slice and serve.