- Preheat oven to 350°F. 
- Warm oil in 10-inch nonstick ovenproof skillet over medium heat. 
- Add leeks; cook 8 minutes, until softened, stirring. 
- Reduce heat to medium low. 
- In medium bowl, whisk eggs, milk, salt, and pepper. 
- Stir in cheese. 
- Pour over leek mixture in skillet. 
- Gently lift up leek mixture to coat bottom of pan. 
- Arrange tomato slices in overlapping pattern on top. 
- Cook 8 minutes, until sides are set, but mixture is still loose on top. 
- Transfer to oven, cook 10 minutes, until center is cooked through and tomatoes are lightly browned. 
- Remove from oven; run a rubber spatula around edges and underneath to loosen frittata from skillet. 
- Slide onto serving platter and sprinkle with Thyme. 
- Cut into wedges. 
- Source: South Beach Diet