Heat the olive oil in a large, deep skillet over medium-high heat. Add the diced onion with salt and pepper and cook for 5 minutes until slightly golden. Cover, reduce the heat to low and cook another 5 minutes until soft and beginning to brown. Stir in the garlic and cook uncovered for 3-4 minutes until fragrant. Add the thyme and cook for 2 minutes. Add the beans and stock, raise the heat and bring to a boil. Reduce to low, cover and simmer for 5 minutes. Uncover and cook 5 more minutes until creamy and the beans are very soft, crushing some beans with a spoon for extra creaminess. Add the liquid smoke, if using. Add the greens and cook until slightly wilted. Stir in the vinegar and season with more salt and pepper.