In a large Dutch oven, cook the bacon over medium-high heat, stirring occasionally, until just crisp, 10 to 15 minutes. Remove the bacon to a paper towel-lined plate. Pour off all but 3 tablespoons of the bacon drippings and lower the heat to medium. Add the onions and cook until tender and translucent, 3 to 5 minutes.
While the bacon and onions are cooking, prepare the greens. To remove the thick, woody stems that run down the center of the leaves, hold the base of the collard stem in one hand. Wrap your other hand around the bottom of the stem, directly under the leaf (like you are holding a baseball bat). In one firm motion, quickly slide your hand up the stem, removing the leaf in one swift movement. To chop the leaves, stack a few on top of each other and roll up like a cigar. Slice the roll into 1-inch pieces, rotate, and give another good chop. Continue until all the greens are chopped.
Increase the heat of the Dutch oven to medium-high. Add a few large handfuls of greens at a time - stirring constantly to allow each batch to wilt for a few moments - until you can fit all of the greens into the pot. Continue to saute the greens for 3 to 5 minutes, until bright green and wilted. Season with a very generous pinch of kosher salt and a few big grinds of freshly ground black pepper. Add the chicken stock, vinegar, sugar, and red pepper flakes to the greens and stir until combined. Bring to a boil, then immediately reduce heat to low. Cover and simmer for 45 minutes.
Add the cooked bacon to the greens and simmer an additional 15 minutes. Taste for seasoning (don't be afraid to be generous with the salt). Serve with hot pepper vinegar on the side.
Cook's Note: The collard greens can be made a few hours ahead of time or up to a day in advance, which will really allow the flavors to deepen. Just keep in the refrigerator until ready to serve. Then reheat over low to medium-low heat until warm.