When garnishing things, I frequently use a form of the main ingredient. In this case, I'd chop some artichoke hearts and, perhaps, arrange them around the edge of the dip, or any-which-way you prefer. The nice thing about using some of the main ingredient as a garnish (aside from cosmetics), is that it gives people an idea of what they're in for. A few broccoli florets floating atop a bowl of cream of broccoli soup; a few thin slices of kielbasa floating atop and bean and chopped kielbasa soup; a few thinly sliced mushrooms atop a mushroom omelet; etc., etc., etc.