Heat oil over medium heat in a large pan. When the oil begins to shimmer, add the onions, garam masala, salt, and chili flakes.
Cook about 5 minutes, or until the spices become fragrant.
Add tomatoes, and cook 2-3 more minutes.
Add the spinach, and cover the pan to let the spinach wilt.
Once the spinach wilts, use an immersion blender or basin blender to blend the onion, tomato, and spinach mixture.
Add the milk and turmeric. Stir.
Add the cubed feta, and cover the pan for 2-3 minutes, until the cheese is warm.
Taste curry. If you prefer it sweeter (like restaurant-style curry), add the maple syrup. Add more salt if bland, or chili flakes if you want it spicier. If it's already too spicy, add a little extra milk.
Serve warm, with rice or naan if desired.