Preheat oven to 400˚F.
Place bacon in an oven-safe dutch oven or large high-sided oven-safe skillet; set skillet over medium heat and cook the bacon slices to a crisp.
Remove bacon from skillet with a slotted spoon and set aside.
Add diced onion to the bacon grease in the skillet and cook for 3 minutes over medium heat.
Stir in garlic and continue to cook for 30 seconds.
Add tomato paste; stir and cook for 1 minute.
Slowly whisk in root beer, apple cider vinegar, maple syrup, and mustard. Bring mixture to a boil.
Gently stir in beans.
Crumble previously cooked bacon and add half of the crumbled bacon to the pot.
Slowly stir mixture and season with salt and pepper; reduce heat to medium low and simmer for 15 minutes.
Transfer beans to the preheated oven and bake, uncovered, for 25 to 30 minutes, or until thickened.
Remove from oven; let stand 5 to 8 minutes to cool slightly.
Top with remaining bacon.
Garnish with red pepper flakes and parsley.