Heat olive oil in a large stock pan. Add onion, salt and pepper and cook for 10 to 12 minutes, until tender. Add quinoa and garlic and stir for one to two minutes. Add in vegetable broth, broccoli and potato. Bring to a simmer and reduce heat to low. Cover and cook for 35 minutes. Remove cover and add spinach, tahini, salt and peppers. Cook until spinach is wilted.
Remove from heat, and allow to cool. When cool enough to handle, transfer to a blender and puree in batches. Return to the pan and heat to low, add cheese and cook until melted.