Bring 2 cups water and quinoa to a boil in a saucepan. Reduce heat to medium-low; cover and simmer until quinoa is tender, 15 to 20 minutes.
Heat oil in a heavy 4-quart pot. Add onion; cook and stir until translucent, about 5 minutes. Stir in vegetable stock, broccoli, sweet potato, and salt. Bring soup to a 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 immersion blender until smooth. Season with nutmeg, garam masala, and pepper.
Combine 1/2 cup water and cashews in a high-speed blender; puree until smooth.
Pour cashew mixture into the soup and bring to a boil.
Place 1/3 cup cooked quinoa in the center of each serving bowl; ladle soup around quinoa.