Combine oil, onion and garlic in a heavy medium-size saucepan over medium-low heat. Cook, stirring, until mixture is fragrant and golden, about 4 to 5 minutes. Add beans and sage; cook, stirring, until hot, about 5 to 6 minutes.
Stir in tuna and cook until just heated through, about 2 minutes. Season to taste with freshly ground white pepper. Garnish with chopped tomatoes and parsley, if desired. Serve immediately.