Place a double layer of paper towels on a heat resistant plate. Heat a large heavy pot with a cover over medium heat. Add bacon and cook until bacon is crisp and brown, stirring occasionally. Remove bacon with a slotted spoon and place on the prepared plate with the paper towel to drain.
Discard all but 1 Tablespoon bacon grease from pot. With pot over medium heat, add the butter and allow pan to reheat. Add the onion slices and cook, stirring occasionally until the onions are translucent with browned edges. Add garlic to pan and cook for two minutes, stirring constantly.
Add vinegar to the pan, stirring to scrape up browned bits. Cook until liquid is reduced by half. Add chard and lightly sprinkle with salt. Stir to combine ingredients. Cover to allow chard to wilt for about 30 seconds.
Uncover and stir, then replace lid for about 30 seconds. Repeat until chard is wilted as desired. Stir in reserved bacon. Taste and adjust seasoning if necessary.
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