In a large pot or Dutch oven, bring 2 quarts water and 1 1/2 teaspoons salt to a boil. Add collard greens and cook until tender, about 10 minutes. Drain collards in a mesh strainer and rinse until cool enough to handle. Add collards to a clean, dry kitchen towel and twist to squeeze excess water from the greens.
In a large skillet over medium heat, cook bacon until crisp, about 8 minutes. Drain bacon on paper towels and reserve bacon fat in pan (there should be about 2 tablespoons. If there is not enough fat, add vegetable oil to reach 2 tablespoons).
Return pan to medium heat, add onion, and cook until softened, about 5 minutes. Stir in garlic and red pepper flakes until fragrant, about 30 seconds.
Add greens to skillet, stirring to coat with bacon fat. Add broth, stir, and cover. Cook until greens are heated through, about 2 minutes. Remove cover and allow to simmer until liquid has thickened, about 3 minutes more.
Remove skillet from heat, top with vinegar and bacon. Season to taste with salt and freshly ground pepper.
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Notes
Bacon: To substitute cooked ham for the bacon, omit Step 2. Cook the onion in 2 tablespoons olive oil or other fat, then add back to the skillet with the collard greens in Step 4.
Apple cider vinegar: Or substitute any other acid such as lemon juice or white vinegar.
Yield: This recipe makes 4 cups collard greens, enough for 4 (1-cup) servings.
Storage: Store leftovers covered in the refrigerator for up to 4 days.