In a large saucepan, cover the quinoa with 6 cups of the water and bring to a boil. Simmer the quinoa over moderate heat until just tender, about 20 minutes. Drain well. Meanwhile, in a mini food processor, process the peanuts until finely ground; do not overprocess.
In a large saucepan, melt the butter in the Annatto Oil. Add the onion and cook over moderate heat, stirring, until softened, about 5 minutes. Add half of the feta cheese and the ground cumin and cook, stirring, for 1 minute. Add the milk, potatoes and the remaining 1 cup of water and season generously with salt. Bring to a boil, then reduce the heat to low and simmer until the potatoes are softened, about 20 minutes. Stir in the chicken stock, heavy cream, quinoa, peanuts and the remaining feta and simmer the soup for 5 minutes. Season the quinoa soup with salt and pepper. Serve at once in warmed soup bowls, passing the Ají at the table.