Step 1Place onion in 13x9-inch baking pan. Drizzle with butter. Toss to mix. Roast, uncovered, at 425 degrees until lightly browned and caramelized, stirring several times, about 30 to 35 minutes. Cool on counter (or put in freezer briefly for a quick cool).
Step 2Place mayonnaise, sour cream, cream cheese, bouillon powder and salt in food processor bowl fitted with metal blade. Mix well. Add onion; pulse on and off until slightly lumpy. An alternative: continuous processing for smooth consistency. Add milk for desired consistency. Refrigerate overnight or as long as 3 days.
Step 3To serve, stir, adjust consistency according to its use and adjust seasoning. Serve chilled.