Preheat the oven to 350F.
Start by peeling the onions, halving them and cutting into thin slices-about a ¼ inch. You don’t want to slice too thinly because the onions will greatly reduce.
In a large saucepan, melt the butter over medium heat. When the butter starts to froth, add the onions and stir to coat. Cook until the onions have softened and lightly caramelize—about 10 minutes. Add the red wine, rosemary and pinch of salt of pepper. Lower the heat to low and cook until the wine has reduced completely.
While the onion combination reduces, heat the honey in a small saucepan over medium heat until it turns a light caramel color-about 2 minutes. Add the red wine vinegar and simmer on low for about 2 minutes. Remove from heat.
When the onion/red wine combination has completely reduced, add in the vinegar/honey combination and stir to combine. Cook for an additional 12-15 minutes or until the mixture reaches a jam like consistency. Remove any large rosemary pieces. Set aside and allow to cool.
To assemble crostini, slice 1 inch rounds out of a baguette. Assemble on rack in oven and bake until toasted-about 5 minutes. Remove bread and slather with goat cheese, spread with onion jam and top with a couple sprigs of arugula. Repeat until you have your desired amount of crostinis. Onion jam will keep refrigerated for one week.