Prepare the leeks. Cut off the ends keeping a little of the light green part, remove outer layers and wash them well. Slice them lengthwise in quarters and finely chop them.
In a medium saucepan, heat the oil and butter in medium heat and when the butter melts add the garlic, spring onion and leeks. Move them around until they are well coated, reduce heat to low and let them sauté until they start softening, for 1-2 minutes.
Slowly add the vegetable stock, increase heat to medium and let it come to a boil. Add the quinoa, slightly stir and let it cook for 15-20 minutes.
Add the lemon juice, chopped parsley, salt and stir slightly. Remove from heat.
Serve the soup topped with the crumbled feta cheese and freshly-ground pepper.