Make dressing: Shake all dressing ingredients together in a medium glass jar. Chill until ready to use.
Make salad: Boil beans until crisp-tender, about 3 minutes. Drain. Scoop into a bowl of ice water to cool; drain again. Cook bacon in a large frying pan over medium heat until crisp, about 10 minutes, and drain on paper towels. Pour out bacon fat and wipe pan clean. Swirl in oil, then add bread cubes and toast, stirring often, until golden brown, about 5 minutes. Drain on paper towels and season with salt and pepper.
Put beans, lettuces, garlic slices, and tarragon leaves in a large bowl and toss with just enough dressing to coat well. Top with bacon and croutons.
*If you're serving this to people with compromised immune systems, use pasteurized yolks (find in the egg case at the grocery store) or omit.
Make-ahead & packing tips: Prep everything up to 1 day ahead. Pack beans in a container; top with lettuces, then garlic and tarragon. Pack bacon in a small container and chill. Store croutons airtight at room temperature, then add in the bacon before leaving home.
Note: Nutritional analysis is per serving.