Begin by trimming the leeks: cut off the root end and the dark green leaves, keeping only the white and light green parts. Slice the leeks lengthwise and then chop into rings. To clean, place the chopped leeks in a bowl of water, swish to remove dirt and sand, then lift out and drain well. In a large skillet, heat olive oil over medium heat. Add the cleaned leeks and sauté for about 5 minutes until they start to soften. Season with salt and pepper. Continue cooking for another 10-15 minutes, stirring occasionally, until the leeks are tender and translucent. You can add water or broth if needed to keep them moist. Remove from heat and let cool slightly before using or storing.