Cook bacon in a large stockpot or Dutch oven over medium heat for 10-12 minutes, or until crisp. Use a slotted spoon to transfer the bacon to a paper towel-lined plate, leaving the drippings in the pot.
Add the onion to the bacon drippings and sauté until translucent, about 7-8 minutes. Stir in garlic and sauté for 1 more minute.
Add the broth, collard greens, vinegar, sugar, salt, and pepper. Stir in about half of the cooked bacon, reserving the other half to use for a garnish at the end. With a wooden spoon, scrape up any browned bits from the bottom of the pot.
Bring the liquid to a boil, then reduce the heat to low and gently simmer (uncovered) for about 1 ½ hours, stirring occasionally (or until collards reach desired level of tenderness). Taste and season with additional salt and pepper, if necessary. Garnish with the reserved crispy bacon, and crushed red pepper flakes or hot sauce.