Heat a large skillet over medium-high heat. Coat pan with cooking spray.
Add leek; saut 5 minutes.
Add garlic; saut 1 minute.
Add beef; cook 7 minutes or until lightly browned, stirring to crumble. Stir in curry paste and tomato sauce; cook until half of liquid evaporates (about 2 minutes).
Add milk and next 4 ingredients (through fish sauce); cook 2 minutes or until slightly thickened.
Serve with the rice and lettuce wedges, if desired.
Garnish with cilantro and green onions, if desired.