Rip the bread into chunks and put it in a blender or food processor using the chopping blade. Blend a few seconds at a time on low. You may need to pour off the crumbs and then return the stubborn chunks to the blender to start over. I was surprised to learn this works quite well even with fresh bread, as long as the bread is reasonably low moisture. Hard crusts will be stubborn, so either cut them off in advance or live with some bigger pieces.
Melt butter in a skillet over medium heat. Add garlic and saute the bread crumbs until golden and fragrant. Add breadcrumbs and stir until toasty. These come out a little soft. They get crisper if you top an oven dish with them. (They're good both soft and crispy.)