Preheat your oven to 350°F (175°C). Grease a 9-inch pie dish or an 8-inch square baking pan.

Heat a tablespoon of olive oil or butter in a skillet over medium heat. Add the diced onion and a pinch of salt. Sauté until soft and just beginning to caramelize, about 6–8 minutes. Remove from heat.

Add spinach to the hot pan and stir until wilted (if using fresh). If using frozen, make sure it’s thawed and squeezed dry before adding. Let the mixture cool slightly.

In a mixing bowl, whisk the 2 eggs with the ½ cup Egg Beaters (or third egg if substituting). Stir in the flour and baking powder until mostly smooth—small lumps are okay. Season with salt and pepper.

Fold the cooled onion and spinach into the egg mixture, then add the crumbled feta and combine evenly.

Pour the batter into the prepared dish and smooth the top.

Bake on the center rack for 30–40 minutes, or until the top is set, slightly golden, and a toothpick inserted near the center comes out clean.

Let the quiche rest for 10–15 minutes before slicing; this helps it finish setting and makes for neater slices.