Rinse potatoes, and place in large saucepan with water to cover.
Cover pan, bring to a boil over high heat and cook until tender, for 12 to 15 minutes.
Remove from heat, and drain.
Meanwhile, heat 2 tablespoons oil in large skillet over medium-high heat, and saute leeks and garlic for 5 to 7 minutes, or until leeks start to brown.
Add steak strips, and saute for 5 minutes more.
Heat remaining 1 tablespoon oil in skillet over medium heat.
Slice potatoes in half, and add to skillet.
Saute for 2 to 3 minutes, or until potatoes start to brown.
Meanwhile, toss steak mixture with shredded cheese and mustard, and season with salt and pepper.
Add potatoes, and place mixture into 2-quart baking dish.
Sprinkle with more freshly ground pepper, if desired.