be onto a winner

To be progressing in a way that is likely to succeed. I think we're onto a winner with this new environmental initiative—the whole company has really embraced it. After all of those failed experiments, our team is finally onto a winner now. Though we sustained heavy losses earlier in the year, we're onto a winner now, thank goodness.

be onto a ˈwinner

(informal) be doing something, especially selling something, that is likely to be successful: He is the sole importer of this product, and he certainly thinks he’s onto a winner.