Warm your olive oil (or bacon fat... or both together) slowly in your biggest skillet over medium heat. Add the diced onions and a big pinch of salt and let them cook, slowly, stirring occasionally, until really very soft and beginning to caramelize. This should take at least 25 minutes. If you have the time, let it take an hour over very low heat.
While the onions cook, rinse and chiffonade your chard. No need to stem the chard, just cut off any grungy ends and slice the stems more closely than the leaves. When you've finished cutting, gather up the whole heap of ribbons and splash them down into a big bowl of cold water. Swirl them about to get off any remaining sand and let them sit until the onions are candy-sweet and very soft.
Once the onions are done, turn up the heat. Then, moving quickly, gather up the greens and carry them, dripping all over your kitchen floor, to the pan. They should sizzle and spit as the water hits the fat. This is OK. Just be careful as you use tongs to carefully turn the whole mess, over and over, until every bit is covered in oil. The water will be steaming and should help wilt the whole giant pile down into something more manageable fairly quickly. Once it's all reduced to a soggy pile, turn the heat down to low and keep turning until the greens are very dark. I can't give you a specific time, I'm afraid, it's very variable. But if it's not dark and tender in about ten minutes, turn up the heat a bit. The whole skillet will fill with a deep red liquid as the chard cooks -- my grandmother used to call this "pot liquor" and it's very good.
Using a garlic press, add as much garlic as you like. I've never found an upper limit for this recipe. A whole head is perfectly reasonable. Stir it in -- it will float in the liquid at the bottom. Let it simmer for about a minute, then add a generous splash of vinegar -- any sort will work here, but I favor cider vinegar in the fall and winter, red wine vinegar in the summer, and chive blossom vinegar in the spring. Taste and season with salt.
OPTIONAL: To make this a one-skillet meal, create several divots in the greens and crack fresh eggs into them. Clamp your lid on and let the eggs poach in the pot liquor for 2-5 minutes, depending on your personal attitudes towards eggs. Served with whole-wheat toast, this is my 8 year old's favorite meal.
If you're not going to add eggs, taste again and see if it needs more oil. It might. It needs a lot of oil You can also top with crumbled sausage or bacon, cooked crispy and diced.