To prepare the greens: cut away the tough stalks and stems from the collards, and discard any leaves that are bruised or yellow. Wash the collards thoroughly to remove the grit, 2 or 3 times, until the water runs clear. Dry thoroughly. Tear the leaves into large pieces.
Place a large pot over medium heat and add the olive oil. Add the onion, garlic, bay leaves, and ham hock. Cook until the onions are soft and starting to brown, about 8 to 10 minutes. Pack in the greens, pushing them down into the pot. Add the broth, vinegar, and sugar. Bring up to a boil turning the greens over occasionally with a wooden spoon as they wilt. Lower to a simmer, cover the pot, and let cook for 45 minutes. Taste the "pot liquor" (broth) and check the seasoning, add salt and pepper. Cover and let cook for 15 more minutes. Remove the bay leaves and serve.