Clean the Leeks: Trim off the roots and the top dark green parts which are tough and not as pleasant to eat. Cut the leeks in half the long way. Place the leeks on their flat sides and slice them into thin half-moons. Add them to a salad spinner (or bowl) with water. Scrunch and agitate them to separate the layers and clean them. Spin or drain and pat dry.
Heat oil and butter in a skillet over medium heat.
Add onion, leeks, and thyme. Sprinkle with salt and cook for 8-10 minutes, stirring often, or until the onions and leeks are soft and browned.
Taste and season to perfection with salt and black pepper and serve.
Video
Notes
Don't skip washing the leeks.Store the cooled leftover sautéed leeks in the fridge in an airtight container for about 3 days. Reheat in the microwave or in a skillet.