Begin by preheating your oven to 400°F (200°C). Prepare a 10-inch quiche or tart pan (a pie plate may also be used) by lightly greasing it with cooking spray or a small amount of oil.

In a medium frying pan, heat a small amount of oil or cooking spray over medium-high heat. Add the diced onion and cook until it becomes translucent and tender, approximately 3-4 minutes. Add the fresh spinach to the pan and cook briefly until it wilts, about 1-2 minutes. Remove from heat and allow the mixture to cool slightly.

In a large mixing bowl, whisk together the eggs, egg substitute, flour, and baking powder until smooth. Gradually whisk in the milk, ensuring the mixture remains free of lumps. Once fully combined, gently fold in the cooled spinach and onion mixture until evenly distributed.

Pour the prepared quiche mixture into the greased pan, ensuring it is evenly spread. Sprinkle the crumbled feta cheese on top of the quiche mixture.

Place the quiche in the preheated oven and bake for approximately 25 minutes, or until the center is set and the edges are golden brown. To check for doneness, the quiche should not jiggle in the center when gently moved.

Once baked, allow the quiche to rest for about 5 minutes before slicing. This will allow the quiche to firm slightly, making it easier to serve. Enjoy warm, or at room temperature.