Melt butter in a pan over medium-low heat. Add onions and let them sit for about 20 minutes as they begin to caramelize. Stir every 10 minutes or so, ensuring they don't turn too dark.
While this dip can be served right away, I typically like to make it either the night before or early in the morning to really give the flavors time to meld. If doing so, wait to toast the bread until right before serving.