Heat the oil in a large saucepan over medium-high heat. Add the garlic and leeks and cook, stirring occasionally, until softened, about 5 minutes. Add the broth, squash, quinoa and raisins and bring to a boil. Reduce the heat and simmer, covered, until the liquid is absorbed and the squash and quinoa are tender, 15 to 20 minutes. Gently stir in the spinach and cook until it just begins to wilt, 2 to 3 minutes. Remove from the heat. Sprinkle with the walnuts.