Combine cream cheese, onion, and garlic: Whisk cream cheese in a medium bowl until smooth. Stir in onion and garlic. Spread mixture evenly onto a serving plate. Chill, uncovered, in refrigerator until firm, about 1 hour.
Let mixture warm slightly: Remove cream cheese mixture from refrigerator and let stand until softened slightly, about 15 minutes.
Make pecan topping: Meanwhile, combine butter, brown sugar, water, Worcestershire sauce, Creole mustard, and cayenne pepper in a small saucepan. Cook over medium, stirring constantly, until thickened and bubbly, 5 to 6 minutes. Remove from heat; stir in pecans. Let stand until cooled slightly, about 5 minutes. Spoon over cream cheese mixture. Serve with buttery crackers.
