How to Make It
Preheat oven to 350°F.
Combine milk, flour, salt, pepper, eggs, and cheese in a medium bowl, stirring with a whisk. Heat oil in a medium ovenproof skillet over medium. Add leek to pan; cook until tender and lightly browned, about 3 minutes. Add garlic; cook 30 seconds. Add egg mixture; stir gently to distribute leek evenly. Cover skillet, and reduce heat to low. Cook until eggs are partially set, about 6 minutes.
Uncover skillet, and bake at 350°F until center is set, 10 to 12 minutes. Loosen frittata from skillet, and slide onto a cutting board. Cut into 8 wedges.