Remove any hard, thick crusts. Tear the bread into smallish pieces. Place in a food processor or mini chopper, and process till coarse breadcrumbs are achieved. Don't worry if not all the crumbs are the same size, this will add to the finished crispy texture.
In a large skillet, heat the olive oil over medium heat. Add the onion and cook, stirring till softened, about five minutes. Sprinkle with a little salt and pepper (the pangrattato also has some salt, you can always add more later). Add in the garlic and cook another minute.
Feel free to use sourdough bread, or ciabatta, whole wheat, etc. Just don't use dried panko style breadcrumbs. These will scorch and get too crispy, and not take on the flavours you are hoping to achieve.
Feel free to add grated parmesan cheese to the finished, cooled pangrattato.
If I am hungry, I can eat an entire can of sardines in this dish by myself. So feel free to use a second can of sardines if desired.