Prep your leeks: discard the top dark green portion of the leeks and keep the bulbs. Slice the bulbs into rings (thick or thin is fine). Add leeks to a colander and rinse them thoroughly (use your hands to dislodge any stubborn dirt – it tends to hide). Shake them a few times so they're not too wet.
Add the oil and butter to a skillet/pan over medium-high heat and let the butter melt.
Stir in the garlic and cook it for about 30 seconds.
Add the leeks to the skillet/pan and toss them until they're completely coated in the butter/oil. Turn the heat down to medium, and sauté them, stirring occasionally, for 7-10 minutes or until they're tender (but not mushy). I recommend tasting them and seeing if they're as soft as you like. Larger leeks tend to be a little tougher.
Season leeks with salt & pepper as needed (I'm generous with both) and serve immediately.