Heat olive oil in a large pan over med heat. Add onions and saute for around 5-7 mins till translucent. Add garlic, basil and salt and mix and saute for 2-3 mins more. Add potatoes and saute 5-6 mins.
Add water and let it simmer for 15-20 mins or until potato pieces are fully cooked. Add red chili flakes and spinach leaves. Bring the gas to low. Stir and cook spinach leaves for 4-5 mins. Turn off the gas. Let it cool down for a minute or two.
Puree soup in an immersion blender or in regular blender.
Once smooth pour it back into the pot. Taste for salt and adjust.
Before serving add lemon juice, red chili flakes and feta cheese on top. Serve warm.