1) To make the dip: Melt the butter in a medium frying pan over moderate heat, and add the sliced onions. Cook, stirring occasionally, until the onions soften, then caramelize. This should take between 10 and 15 minutes. The lower the heat, the longer it takes, but the less likely you are to burn the onions.
2) Once the onions are a medium brown, remove from the heat and add the vinegar.
3) Place the onions and vinegar into a small food processor. Add the honey and mustard, and process or blend until smooth.
4) Add the mayonnaise and salt and pepper to taste, stirring to combine. Refrigerate, covered, until ready to serve.
My Note: I added a little garlic and onion powder because we like the flavor. The second time I made it, I decided to add 2 cloves of garlic, and it turned out even tastier than the first time.
I served the dip along with some raw carrots as well baked zucchini sticks were tasty and the onion dip was a hit, and it was well liked. It took closer to 45 minutes for the onions to be caramelized. At 10-15 minutes the onions weren't even light brown yet. In caramelizing onions patience seems to be is a virtue. Perhaps I missed a secret step along the way.