In a small bowl, combine 1/4 cup olive oil, red wine vinegar, Dijon mustard, and minced garlic; stir well with a whisk. Add 1/4 cup thinly sliced red onion; toss and let stand 5 minutes. Season with kosher salt and freshly ground black pepper, to taste. In a large bowl, combine vinaigrette and cannellini beans. Purée 1/3 of the mixture in a mini-food processor until smooth. Fold mixture back into beans; add tuna and 1/4 cup olive oil. Stir in 2 tablespoons sliced mint. Mix until tuna is no longer chunky. Toast Italian bread in a toaster or oven until golden and crispy; cut in half. Arrange bread on a platter. Spoon about 2 tablespoons tuna-and-bean mixture onto each slice. Garnish with 1/4 cup thinly sliced red onion and 1 tablespoon sliced mint.