Trim the roots and dark green tops from the leeks. Cut them in half lengthwise and rinse thoroughly under cold water to remove any dirt. Slice into half-moons.
In a large saucepan, melt the butter over medium heat. Add the sliced leeks and cook for about 5-7 minutes, stirring occasionally, until they are tender.
Once the leeks are softened, sprinkle the flour over them and stir for another minute. Gradually whisk in the milk, continuing to stir until the mixture thickens (about 5 minutes).
Lower the heat and add the cheddar cheese, Parmesan cheese, garlic powder, salt, and pepper. Stir until the cheese is melted and the sauce is smooth.
Pour the cheesy leek mixture into a baking dish, spreading it evenly.
Preheat the oven to 375°F (190°C). Bake the leeks for 15 minutes, or until the top is golden and bubbly.
Remove from the oven, allow to cool slightly, and garnish with fresh parsley if desired. Serve warm as a side dish.