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