1. Sauté the garlic, onions, and celery on a low flame for 15 to 20 minutes until very cooked through and melting together.
2. Add the pastrami, and the salt and pepper to taste. Sauté for a few more minutes, then add half a cup white wine to deglaze the pan. Make sure to scrape up all the burnt bits in there.
3. Add the cup of mashed potatoes and mix together until it becomes a filling. Stuff the capons with the filling.