Preheat oven to 350º.
Rinse hens with cold water; pat dry. Remove skin; trim excess fat. Split hens in half lengthwise. Combine flour, salt, and pepper in a shallow dish; dredge hens in flour mixture. Melt butter in a large Dutch oven over medium-high heat. Add oil and hens, and cook 4 minutes on each side or until browned. Remove from pan.
Add leek, mushrooms, and garlic to pan; sauté 5 minutes. Add broth and vinegar, stirring to loosen browned bits. Return hens to pan; add tomatoes, apple, and potatoes. Bring to a simmer. Cover and bake at 350º for 30 minutes or until hens are done. Place hens on a serving platter, and keep warm. Remove vegetable mixture from pan with a slotted spoon; place in a serving bowl. Reserve 2 cups cooking liquid in pan; discard remaining liquid. Bring reserved cooking liquid to a boil; cook 8 minutes or until reduced by half, stirring occasionally. Pour over vegetable mixture. Sprinkle with parsley. Serve with hens.