Heat 1 tablespoon of olive oil in a medium skillet over medium-high heat.
Add the bread crumbs and cook, stirring frequently, until they are golden brown, about 4–5 minutes.
Remove the skillet from the heat and stir in the minced garlic and a pinch of salt and pepper; set the toasted breadcrumbs aside to cool.
Make the vinaigrette: combine the lemon juice, whole-grain mustard, Dijon mustard, honey, 1/2 teaspoon kosher salt, 1/4 teaspoon black pepper, and 1/3 cup olive oil in a jar or small bowl; shake or whisk until emulsified.
In a large bowl, gently toss the drained cannellini beans, chopped parsley, chopped celery, chopped red onion, and crumbled tuna until well combined.
Pour the vinaigrette over the tuna and bean mixture and toss gently to coat evenly.
Arrange the butter lettuce leaves on plates or a serving platter and spoon the tuna and bean salad on top.
Sprinkle the toasted breadcrumbs over the salad and serve immediately.
Optional appetizer: toast 1/2-inch diagonal baguette slices brushed lightly with olive oil until golden, then top with the salad and breadcrumbs and serve at once.