Trim away any tough stems, then wash the greens: Rinse each leaf separately. Put all of them in a sinkful of water and let stand for 5 minutes. Now, lift them out of the sink and drain away the water and all the grit that has settled to the bottom. Repeat the sink bath until you don't see any grit when you remove the leaves. Chop the onions and crush the garlic cloves. In a heavy pot large enough to hold all the greens, warm the oil over medium heat. Set aside about half of the onions and add the remainder along with the garlic to the pot. Saute until softened about 5 minutes. Add the greens, turn to coat in the oil, and then pour in the stock to cover. Bring to a boil and season with salt and pepper. Reduce the heat to low, cover, and cook until the greens are tender. The timing will depend on what you are cooking; collards can take up to 40 minutes or so, while beet greens will cook in 1/2 that time. Transfer to a serving bowl. Pass the reserved onion and pepper vinegar at the table for sprinkling over the top.