sauté onion in oil in a pan over medium heat until soft, 3 to 5 minutes. Add garlic and Rosemary. Cook until fragrant, about one minute.
Stir in beans, 1/2 cup reserved liquid, fontina, olives, salt, and pepper. Simmer mixture over low heat, stirring often, until cheese melts and flavors blend, about 20 minutes. Add more reserved liquid to loosen beans if needed. Remove beans from heat and stir in parsley. Makes 3 cups.