Heat a large skillet over medium-high heat. Coat pan with cooking spray. Add leek; sauté 5 minutes. Add garlic; sauté 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.