The phrase "be onto a winner" means to be in a position of success or likely to succeed. There are several synonyms for this phrase that can be used interchangeably depending on the context. For example, the phrase "hit the jackpot" conveys the same idea of finding success or fortune. Another synonym is "strike gold," which implies a fortunate discovery or achievement. "Be in the money" is another phrase that conveys the idea of being successful financially. Other synonyms include "have a winning streak" or "be on a roll," which imply ongoing success or winning. Overall, there are many ways to express the idea of success, each with its unique connotation.