In a medium skillet, heat the extra-virgin olive oil over medium-high heat. Add the bread crumbs and cook, stirring frequently, until crumbs begin to brown, about 4 to 5 minutes. Remove from the heat and stir in the garlic and a pinch of Kosher salt and black pepper each.
To make the vinaigrette, place the lemon juice, whole-grain mustard, Dijon mustard, honey, Kosher salt, black pepper, and extra-virgin olive oil in a jar. Affix the lid and shake vigorously until emulsified.
In a large bowl, gently mix together the cannellini beans, flat-leaf parsley, celery, onion, and tuna. Pour the vinaigrette over the mixture and toss gently to coat.
Arrange the butter lettuce on individual plates or one large serving platter and top with the tuna and bean mixture. Sprinkle the toasted bread crumbs over the top and serve.
To serve as an appetizer, cut ½-inch slices on a diagonal from a baguette. Brush olive oil over one side of the slice and then toast until lightly brown. Top with the salad and bread crumbs. Serve at once.
