Combine first 4 ingredients, stirring well with a whisk. Cover and chill.
Secure beef at 2-inch intervals with twine. Sprinkle beef with pepper. Heat a large nonstick skillet over medium-high heat; coat pan with cooking spray. Add beef to pan; cook 15 minutes or until desired degree of doneness, turning frequently. Let stand 15 minutes. Cut into 16 slices. Sprinkle with lemon juice.
Arrange watercress evenly on bread slices. Place 1 beef slice and about 1 tablespoon chilled sauce over each bread slice. Arrange capers and cheese evenly over sauce.
"While I was visiting a friend, she prepared these mini open-faced sandwiches. They make delightful appetizers for holiday guests, and the Mustard-Horseradish Sauce can be made ahead of time." -Susan Wambui Thiong'o, Nairobi, Kenya