Preheat oven to 450 degrees. Place slices on a baking sheet. Brush top side with olive oil; sprinkle with salt and pepper. Bake until golden, 4 to 6 minutes. Set aside.
In a large bowl, whisk together lemon juice, 2 tablespoons mustard, and reserved tuna oil; season with salt and pepper. Add tuna, beans, parsley, shallot, and celery; toss to combine.
Spread bread with remaining 2 tablespoons mustard. Top with tuna salad, and serve.