makes one round, about 10-inches diameter; about 6 servings
Preheat oven to 350° F. Heat olive oil in a frying pan over medium heat. Add onions and cook, stirring occasionally, until caramelized, about 10 minutes. Add the garlic, a few grinds of fresh black pepper, and cook a couple minutes longer. Add the greens and cover for a minute to wilt. Remove from heat and let cool while you proceed.
In a medium bowl, whisk eggs with the milk to combine. Add the cooked quinoa and cheese. Stir in the cooked greens, onions, and garlic. Grease interior of a glass pie plate (in the ballpark of 10-inches) with butter or olive oil. Pour in quiche mixture and smooth into an even layer.
Put in oven, reduce temperature to 325° F and bake for 50 minutes. Raise temperature back to 350 and bake 5 more minutes to brown the top. Let cool 5 minutes before slicing. Serve warm or at room temperature. Refrigerate any leftovers.