Preheat oven to 350 F.

Warm olive oil in a 10-inch nonstick ovenproof skillet over medium heat.

Add leeks to the skillet and cook for 8 minutes, stirring occasionally, until softened.

Reduce heat to medium-low.

In a medium bowl, whisk together eggs, milk, salt, and pepper.

Stir in shredded cheddar cheese.

Pour the egg mixture over the leeks in the skillet.

Gently lift up the leek mixture to coat the bottom of the pan.

Arrange tomato slices in an overlapping pattern on top of the egg mixture.

Cook on the stovetop for 8 minutes, until the sides are set but the mixture is still loose on top.

Transfer the skillet to the preheated oven.

Bake for 10 minutes, until the center is cooked through and the tomatoes are lightly browned.

Remove the frittata from the oven.

Run a rubber spatula around the edges and underneath to loosen the frittata from the skillet.

Slide the frittata onto a serving platter.

Sprinkle with fresh thyme.

Cut into wedges and serve.