Preheat oven to 350 degrees F. Grease a quiche pan, pie dish, or other baking dish with olive oil or butter.
In a medium bowl, mix together the eggs, milk, whole wheat flour, and baking powder. Season with salt and pepper.
Add the onions, feta, and spinach to the bowl and stir to combine.
Pour mixture into prepared quiche pan.
Top the quiche carefully with tomatoes.
Place in preheated oven and bake for 60 minutes, or until quiche is set.
Allow to cool for at least 10 minutes before slicing.
Serve warm, at room temperature, or cold from the fridge.