Bring water and quinoa to the boil in a saucepan. Reduce heat to medium-low; cover and simmer until quinoa is tender, 15 to 20 minutes.
Heat oil in a soup pot. Add onion; cook and stir until softened, about 5 minutes. Stir in vegetable stock, broccoli, sweet potato and salt. Bring soup to the boil; reduce heat and simmer until broccoli and sweet potato are very tender, 15 to 20 minutes.
Remove soup from the heat. Puree with a hand-held stick blender until smooth. Season with nutmeg, garam masala and pepper.
Combine 120ml water and cashews in a high-speed blender; puree until smooth.
Pour cashew mixture into the soup and bring to the boil.
Place 1/4 of the cooked quinoa in the centre of each serving bowl; ladle soup around quinoa.
The quinoa complements the soup well if made with vegetable stock instead of water.