Coat a large saucepan with cooking spray and heat over medium-high heat. Stir-fry onions, potatoes, garlic and coriander seeds for a few minutes. Add a couple tablespoons of water, reduce heat to low and cover pan.
Leave to 'sweat' until onions are soft, about 15 minutes.
Add broth, spinach and cilantro; cook for 10 minutes more. Puree soup in pan with an immersion blender (or puree in batches in a blender but be careful not to splatter hot liquid). Season to taste and serve.