Steam the potatoes for about 10 minutes (until tender when pricked with a fork).
While the potatoes are steaming, cook the bacon in a large skillet until crispy (about 8 - 10 minutes).
When the potatoes are tender transfer to a medium bowl and cover with foil to keep warm.
Remove the bacon and place on a paper towel to drain. Use a slotted spoon and leave the bacon grease in the pan.
Add the shallot and garlic to the pan and cook in the bacon grease until softened (about 1 minute).
Whisk in vinegar, mustard, sugar and salt. Cook until mixture is thickened and reduced by half, about 5 minutes.
Remove the skillet from the heat. Taste and adjust seasonings with salt and pepper.
Cut each fingerling potato crosswise at a forty-five degree angle into ½ inch thick slices.
Add the potatoes to the skillet and toss to coat with dressing.
Add the spinach, herbs, and cooked bacon and gently toss to coat with dressing.
Taste and adjust seasonings once more with salt and pepper.