Preheat your oven to 180°C (350°F).
In a pan, sauté the chopped onion until translucent. Add the chopped spinach and cook for 1–2 minutes just to wilt it. Remove from heat and set aside.
In a large bowl, whisk together the eggs, salt, pepper, garlic powder, oil, milk, and cream cheese until well combined.
Stir in the flour and baking powder to form a smooth, thick batter.
Fold in the sautéed spinach and onion mixture.
Pour the batter into a greased baking dish and smooth the top.
Place triangular cheese slices on top, pressing them lightly into the surface.
Bake for 25–30 minutes or until golden brown and a toothpick inserted comes out clean.
Let the casserole rest for 5–10 minutes before slicing. Serve warm or at room temperature.