1. Rinse quinoa thoroughly in cold water. Drain well.
2. In a large skillet, warm oil over medium-high heat. Add onion and cook, stirring often, until softened, 3 to 5 minutes. Stir in tomatoes and broth and increase heat to high. Bring to a boil, then carefully transfer to slow cooker. Stir in quinoa and season with salt and pepper. Cover and cook on low until most of the liquid is absorbed and quinoa is tender, about 2 hours.
3. Place pine nuts in a small skillet over medium-low heat. Cook, shaking skillet constantly, until pine nuts are light golden and fragrant, about 3 minutes. Immediately transfer to a bowl.
4. Remove slow cooker cover and stir in spinach and pine nuts. (If pilaf looks dry, add up to 1/2 cup more broth.) Cover and cook 15 minutes longer, or until the spinach has wilted completely. Season with additional salt and pepper and serve.