Preheat oven to 350 degrees F. Oil a 2-quart gratin dish.
In a saute pan over medium heat, heat olive oil and add leeks. Saute, stirring, until they become tender, about 5 minutes. Add the garlic and continue to cook, stirring, for another minute. Add the peppers and cook, covered, for 5 to 10 more minutes, until peppers are tender, yet crisp. Remove pepper mixture for the heat.
In a bowl, beat eggs and milk. Stir in the quinoa and add cheese, potatoes, pepper mixture, salt, pepper and thyme. Pour into the prepared baking dish and bake for about 35 to 45 minutes, until the top is golden brown. Let mixture rest about 5 minutes before serving