Lightly grease a 10-inch quiche or tart pan (or a deep dish pie plate)
Heat the olive oil over medium high heat in a skillet, and saute the onion until softened.
Add in the spinach and cook until just wilted (you may need to do this in batches if your skillet is too small). Season with salt and pepper to taste. Set aside to cool 5 minutes.
Whisk together eggs, flour, and baking powder. Whisk in milk, then stir in spinach-onion mixture. Add pepper, mix well.
Pour egg mixture into quiche pan. Top with feta.
Place the tomato halves on top, and press gently so only the surface is visible.
Bake for 25-30 minutes, or until center is set and the edge is golden brown - and pulling away from the pan.
Cool for 10 minutes.
Garnish with extra feta, and some basil (optional).
Slice, and serve.
This crustless quiche tastes great hot, room temp, OR chilled! You'll notice there is no salt and pepper in this - the feta is plenty salty enough, so there's no need to add it. You may enjoy some pepper in yours though!